[SPARK-28787][DOC][SQL] Document LOAD DATA statement in SQL Reference

### What changes were proposed in this pull request?
Document LOAD DATA statement in SQL Reference

### Why are the changes needed?
To complete the SQL Reference

### Does this PR introduce any user-facing change?
Yes

### How was this patch tested?
Tested using jykyll build --serve

Here are the screen shots:

![image](https://user-images.githubusercontent.com/13592258/64073167-e7cd0800-cc4e-11e9-9fcc-92fe4cb5a942.png)

![image](https://user-images.githubusercontent.com/13592258/64073169-ee5b7f80-cc4e-11e9-9a36-cc023bcd32b1.png)

![image](https://user-images.githubusercontent.com/13592258/64073170-f4516080-cc4e-11e9-9101-2609a01fe6fe.png)

Closes apache#25522 from huaxingao/spark-28787.

Authored-by: Huaxin Gao <[email protected]>
Signed-off-by: Sean Owen <[email protected]>

docs/sql-ref-syntax-dml-load.md

@@ -1,7 +1,7 @@
 ---
 layout: global
-title: LOAD
-displayTitle: LOAD
+title: LOAD DATA
+displayTitle: LOAD DATA
 license: |
   Licensed to the Apache Software Foundation (ASF) under one or more
   contributor license agreements.  See the NOTICE file distributed with
@@ -19,4 +19,101 @@ license: |
   limitations under the License.
 ---
 
-**This page is under construction**
+### Description
+`LOAD DATA` statement loads the data into a table from the user specified directory or file. If a directory is specified then all the files from the directory are loaded. If a file is specified then only the single file is loaded. Additionally the `LOAD DATA` statement takes an optional partition specification. When a partition is specified, the data files (when input source is a directory) or the single file (when input source is a file) are loaded into the partition of the target table.
+
+### Syntax
+{% highlight sql %}
+LOAD DATA [ LOCAL ] INPATH path [ OVERWRITE ] INTO TABLE table_name
+  [ PARTITION ( partition_col_name = partition_col_val [ , ... ] ) ]
+{% endhighlight %}
+
+### Parameters
+<dl>
+  <dt><code><em>path</em></code></dt>
+  <dd>Path of the file system. It can be either an absolute or a relative path.</dd>
+</dl>
+
+<dl>
+  <dt><code><em>table_name</em></code></dt>
+  <dd>The name of an existing table.</dd>
+</dl>
+
+<dl>
+  <dt><code><em>PARTITION ( partition_col_name = partition_col_val [ , ... ] )</em></code></dt>
+  <dd>Specifies one or more partition column and value pairs.</dd>
+</dl>
+
+<dl>
+  <dt><code><em>LOCAL</em></code></dt>
+  <dd>If specified, it causes the <code>INPATH</code> to be resolved against the local file system, instead of the default file system, which is typically a distributed storage.</dd>
+</dl>
+
+<dl>
+  <dt><code><em>OVERWRITE</em></code></dt>
+  <dd>By default, new data is appended to the table. If <code>OVERWRITE</code> is used, the table is instead overwritten with new data.</dd>
+</dl>
+
+### Examples
+{% highlight sql %}
+ -- Example without partition specification.
+ -- Assuming the students table has already been created and populated.
+ SELECT * FROM students;
+
+     + -------------- + ------------------------------ + -------------- +
+     | name           | address                        | student_id     |
+     + -------------- + ------------------------------ + -------------- +
+     | Amy Smith      | 123 Park Ave, San Jose         | 111111         |
+     + -------------- + ------------------------------ + -------------- +
+
+ CREATE TABLE test_load (name VARCHAR(64), address VARCHAR(64), student_id INT);
+
+ -- Assuming the students table is in '/user/hive/warehouse/'
+ LOAD DATA LOCAL INPATH '/user/hive/warehouse/students' OVERWRITE INTO TABLE test_load;
+
+ SELECT * FROM test_load;
+
+     + -------------- + ------------------------------ + -------------- +
+     | name           | address                        | student_id     |
+     + -------------- + ------------------------------ + -------------- +
+     | Amy Smith      | 123 Park Ave, San Jose         | 111111         |
+     + -------------- + ------------------------------ + -------------- +
+
+ -- Example with partition specification.
+ CREATE TABLE test_partition (c1 INT, c2 INT, c3 INT) USING HIVE PARTITIONED BY (c2, c3);
+
+ INSERT INTO test_partition PARTITION (c2 = 2, c3 = 3) VALUES (1);
+
+ INSERT INTO test_partition PARTITION (c2 = 5, c3 = 6) VALUES (4);
+
+ INSERT INTO test_partition PARTITION (c2 = 8, c3 = 9) VALUES (7);
+
+ SELECT * FROM test_partition;
+
+      + ------- + ------- + ----- +
+      | c1      | c2      | c3    |
+      + ------- + --------------- +
+      | 1       | 2       | 3     |
+      + ------- + ------- + ----- +
+      | 4       | 5       | 6     |
+      + ------- + ------- + ----- +
+      | 7       | 8       | 9     |
+      + ------- + ------- + ----- +
+
+ CREATE TABLE test_load_partition (c1 INT, c2 INT, c3 INT) USING HIVE PARTITIONED BY (c2, c3);
+
+ -- Assuming the test_partition table is in '/user/hive/warehouse/'
+ LOAD DATA LOCAL INPATH '/user/hive/warehouse/test_partition/c2=2/c3=3'
+     OVERWRITE INTO TABLE test_load_partition PARTITION (c2=2, c3=3);
+
+ SELECT * FROM test_load_partition;
+
+      + ------- + ------- + ----- +
+      | c1      | c2      | c3    |
+      + ------- + --------------- +
+      | 1       | 2       | 3     |
+      + ------- + ------- + ----- +
+
+
+{% endhighlight %}
+