[SPARK-38800][DOCS][PYTHON] Explicitly document the supported pandas version

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

This PR proposes to document the supported pandas version for pandas API on Spark.

### Why are the changes needed?

Since the behavior of pandas is different per its version, it would be better explicitly documenting the supported pandas version so that users won't confuse.

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

Yes, now the supported pandas version is mentioned to the PySpark API reference page as below:

<img width="1109" alt="Screen Shot 2022-04-08 at 10 35 28 AM" src="https://user-images.githubusercontent.com/44108233/162345946-87d5168c-8ff2-40f3-a0d3-2979332e67b7.png">

### How was this patch tested?

This existing doc build should cover

Closes #36095 from itholic/SPARK-38800/supported_pandas_version.

Lead-authored-by: itholic <[email protected]>
Co-authored-by: Hyukjin Kwon <[email protected]>
Signed-off-by: Hyukjin Kwon <[email protected]>

Author: itholic

python/docs/source/reference/index.rst

@@ -22,6 +22,8 @@ API Reference
 
 This page lists an overview of all public PySpark modules, classes, functions and methods.
 
+Pandas API on Spark follows the API specifications of latest pandas release.
+
 .. toctree::
    :maxdepth: 2