[SPARK-22110][SQL][DOCUMENTATION] Add usage and improve documentation…

… with arguments and examples for trim function

## What changes were proposed in this pull request?

This PR proposes to enhance the documentation for `trim` functions in the function description session.

- Add more `usage`, `arguments` and `examples` for the trim function
- Adjust space in the `usage` session

After the changes, the trim function documentation will look like this:

- `trim`

```trim(str) - Removes the leading and trailing space characters from str.

trim(BOTH trimStr FROM str) - Remove the leading and trailing trimStr characters from str

trim(LEADING trimStr FROM str) - Remove the leading trimStr characters from str

trim(TRAILING trimStr FROM str) - Remove the trailing trimStr characters from str

Arguments:

str - a string expression
trimStr - the trim string characters to trim, the default value is a single space
BOTH, FROM - these are keywords to specify trimming string characters from both ends of the string
LEADING, FROM - these are keywords to specify trimming string characters from the left end of the string
TRAILING, FROM - these are keywords to specify trimming string characters from the right end of the string
Examples:

> SELECT trim('    SparkSQL   ');
 SparkSQL
> SELECT trim('SL', 'SSparkSQLS');
 parkSQ
> SELECT trim(BOTH 'SL' FROM 'SSparkSQLS');
 parkSQ
> SELECT trim(LEADING 'SL' FROM 'SSparkSQLS');
 parkSQLS
> SELECT trim(TRAILING 'SL' FROM 'SSparkSQLS');
 SSparkSQ
```

- `ltrim`

```ltrim

ltrim(str) - Removes the leading space characters from str.

ltrim(trimStr, str) - Removes the leading string contains the characters from the trim string

Arguments:

str - a string expression
trimStr - the trim string characters to trim, the default value is a single space
Examples:

> SELECT ltrim('    SparkSQL   ');
 SparkSQL
> SELECT ltrim('Sp', 'SSparkSQLS');
 arkSQLS
```

- `rtrim`
```rtrim

rtrim(str) - Removes the trailing space characters from str.

rtrim(trimStr, str) - Removes the trailing string which contains the characters from the trim string from the str

Arguments:

str - a string expression
trimStr - the trim string characters to trim, the default value is a single space
Examples:

> SELECT rtrim('    SparkSQL   ');
 SparkSQL
> SELECT rtrim('LQSa', 'SSparkSQLS');
 SSpark
```

This is the trim characters function jira: [trim function](https://issues.apache.org/jira/browse/SPARK-14878)

## How was this patch tested?

Manually tested
```
spark-sql> describe function extended trim;
17/09/22 17:03:04 INFO CodeGenerator: Code generated in 153.026533 ms
Function: trim
Class: org.apache.spark.sql.catalyst.expressions.StringTrim
Usage:
    trim(str) - Removes the leading and trailing space characters from `str`.

    trim(BOTH trimStr FROM str) - Remove the leading and trailing `trimStr` characters from `str`

    trim(LEADING trimStr FROM str) - Remove the leading `trimStr` characters from `str`

    trim(TRAILING trimStr FROM str) - Remove the trailing `trimStr` characters from `str`

Extended Usage:
    Arguments:
      * str - a string expression
      * trimStr - the trim string characters to trim, the default value is a single space
      * BOTH, FROM - these are keywords to specify trimming string characters from both ends of
          the string
      * LEADING, FROM - these are keywords to specify trimming string characters from the left
          end of the string
      * TRAILING, FROM - these are keywords to specify trimming string characters from the right
          end of the string

    Examples:
      > SELECT trim('    SparkSQL   ');
       SparkSQL
      > SELECT trim('SL', 'SSparkSQLS');
       parkSQ
      > SELECT trim(BOTH 'SL' FROM 'SSparkSQLS');
       parkSQ
      > SELECT trim(LEADING 'SL' FROM 'SSparkSQLS');
       parkSQLS
      > SELECT trim(TRAILING 'SL' FROM 'SSparkSQLS');
       SSparkSQ
  ```
```
spark-sql> describe function extended ltrim;
Function: ltrim
Class: org.apache.spark.sql.catalyst.expressions.StringTrimLeft
Usage:
    ltrim(str) - Removes the leading space characters from `str`.

    ltrim(trimStr, str) - Removes the leading string contains the characters from the trim string

Extended Usage:
    Arguments:
      * str - a string expression
      * trimStr - the trim string characters to trim, the default value is a single space

    Examples:
      > SELECT ltrim('    SparkSQL   ');
       SparkSQL
      > SELECT ltrim('Sp', 'SSparkSQLS');
       arkSQLS

```

```
spark-sql> describe function extended rtrim;
Function: rtrim
Class: org.apache.spark.sql.catalyst.expressions.StringTrimRight
Usage:
    rtrim(str) - Removes the trailing space characters from `str`.

    rtrim(trimStr, str) - Removes the trailing string which contains the characters from the trim string from the `str`

Extended Usage:
    Arguments:
      * str - a string expression
      * trimStr - the trim string characters to trim, the default value is a single space

    Examples:
      > SELECT rtrim('    SparkSQL   ');
       SparkSQL
      > SELECT rtrim('LQSa', 'SSparkSQLS');
       SSpark

```

Author: Kevin Yu <[email protected]>

Closes apache#19329 from kevinyu98/spark-14878-5.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/stringExpressions.scala

@@ -535,20 +535,36 @@ object StringTrim {
 @ExpressionDescription(
   usage = """
     _FUNC_(str) - Removes the leading and trailing space characters from `str`.
-    _FUNC_(BOTH trimStr FROM str) - Remove the leading and trailing trimString from `str`
+
+    _FUNC_(BOTH trimStr FROM str) - Remove the leading and trailing `trimStr` characters from `str`
+
+    _FUNC_(LEADING trimStr FROM str) - Remove the leading `trimStr` characters from `str`
+
+    _FUNC_(TRAILING trimStr FROM str) - Remove the trailing `trimStr` characters from `str`
   """,
   arguments = """
     Arguments:
       * str - a string expression
-      * trimString - the trim string
-      * BOTH, FROM - these are keyword to specify for trim string from both ends of the string
+      * trimStr - the trim string characters to trim, the default value is a single space
+      * BOTH, FROM - these are keywords to specify trimming string characters from both ends of
+          the string
+      * LEADING, FROM - these are keywords to specify trimming string characters from the left
+          end of the string
+      * TRAILING, FROM - these are keywords to specify trimming string characters from the right
+          end of the string
   """,
   examples = """
     Examples:
       > SELECT _FUNC_('    SparkSQL   ');
        SparkSQL
+      > SELECT _FUNC_('SL', 'SSparkSQLS');
+       parkSQ
       > SELECT _FUNC_(BOTH 'SL' FROM 'SSparkSQLS');
        parkSQ
+      > SELECT _FUNC_(LEADING 'SL' FROM 'SSparkSQLS');
+       parkSQLS
+      > SELECT _FUNC_(TRAILING 'SL' FROM 'SSparkSQLS');
+       SSparkSQ
   """)
 case class StringTrim(
     srcStr: Expression,
@@ -634,13 +650,13 @@ object StringTrimLeft {
 @ExpressionDescription(
   usage = """
     _FUNC_(str) - Removes the leading space characters from `str`.
+
     _FUNC_(trimStr, str) - Removes the leading string contains the characters from the trim string
   """,
   arguments = """
     Arguments:
       * str - a string expression
-      * trimString - the trim string
-      * BOTH, FROM - these are keyword to specify for trim string from both ends of the string
+      * trimStr - the trim string characters to trim, the default value is a single space
   """,
   examples = """
     Examples:
@@ -735,13 +751,13 @@ object StringTrimRight {
 @ExpressionDescription(
   usage = """
     _FUNC_(str) - Removes the trailing space characters from `str`.
+
     _FUNC_(trimStr, str) - Removes the trailing string which contains the characters from the trim string from the `str`
   """,
   arguments = """
     Arguments:
       * str - a string expression
-      * trimString - the trim string
-      * BOTH, FROM - these are keyword to specify for trim string from both ends of the string
+      * trimStr - the trim string characters to trim, the default value is a single space
   """,
   examples = """
     Examples: