[SPARK-32088][PYTHON] Pin the timezone in timestamp_seconds doctest

### What changes were proposed in this pull request?

Add American timezone during timestamp_seconds doctest

### Why are the changes needed?

`timestamp_seconds` doctest in `functions.py` used default timezone to get expected result
For example:

```python
>>> time_df = spark.createDataFrame([(1230219000,)], ['unix_time'])
>>> time_df.select(timestamp_seconds(time_df.unix_time).alias('ts')).collect()
[Row(ts=datetime.datetime(2008, 12, 25, 7, 30))]
```

But when we have a non-american timezone, the test case will get different test result.

For example, when we set current timezone as `Asia/Shanghai`, the test result will be

```
[Row(ts=datetime.datetime(2008, 12, 25, 23, 30))]
```

So no matter where we run the test case ,we will always get the expected permanent result if we set the timezone on one specific area.

### Does this PR introduce _any_ user-facing change?

No

### How was this patch tested?

Unit test

Closes apache#28932 from GuoPhilipse/SPARK-32088-fix-timezone-issue.

Lead-authored-by: GuoPhilipse <[email protected]>
Co-authored-by: GuoPhilipse <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: GuoPhilipse

python/pyspark/sql/functions.py

@@ -1431,9 +1431,11 @@ def to_utc_timestamp(timestamp, tz):
 def timestamp_seconds(col):
     """
     >>> from pyspark.sql.functions import timestamp_seconds
+    >>> spark.conf.set("spark.sql.session.timeZone", "America/Los_Angeles")
     >>> time_df = spark.createDataFrame([(1230219000,)], ['unix_time'])
     >>> time_df.select(timestamp_seconds(time_df.unix_time).alias('ts')).collect()
     [Row(ts=datetime.datetime(2008, 12, 25, 7, 30))]
+    >>> spark.conf.unset("spark.sql.session.timeZone")
     """
 
     sc = SparkContext._active_spark_context

sql/core/src/main/scala/org/apache/spark/sql/functions.scala

@@ -3360,8 +3360,8 @@ object functions {
 
   /**
    * Creates timestamp from the number of seconds since UTC epoch.
-   * @group = datetime_funcs
-   * @since = 3.1.0
+   * @group datetime_funcs
+   * @since 3.1.0
    */
   def timestamp_seconds(e: Column): Column = withExpr {
     SecondsToTimestamp(e.expr)