[SPARK-33256][PYTHON][DOCS] Clarify PySpark follows NumPy documentati…

…on style

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

This PR adds few lines about docstring style to document that PySpark follows [NumPy documentation style](https://numpydoc.readthedocs.io/en/latest/format.html). We all completed the migration to NumPy documentation style at SPARK-32085.

Ideally we should have a page like https://pandas.pydata.org/docs/development/contributing_docstring.html but I would like to leave it as a future work.

### Why are the changes needed?

To tell developers that PySpark now follows NumPy documentation style.

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

No, it's a change in unreleased branches yet.

### How was this patch tested?

Manually tested via `make clean html` under `python/docs`:

![Screen Shot 2020-12-06 at 1 34 50 PM](https://user-images.githubusercontent.com/6477701/101271623-d5ce0380-37c7-11eb-93ac-da73caa50c37.png)

Closes #30622 from HyukjinKwon/SPARK-33256.

Authored-by: HyukjinKwon <gurwls223@apache.org>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>

python/docs/source/development/contributing.rst

@@ -123,11 +123,12 @@ Annotations can be validated using ``dev/lint-python`` script or by invoking myp
 
 
 
-Code Style Guide
-----------------
+Code and Docstring Guide
+----------------------------------
 
 Please follow the style of the existing codebase as is, which is virtually PEP 8 with one exception: lines can be up
 to 100 characters in length, not 79.
+For the docstring style, PySpark follows `NumPy documentation style <https://numpydoc.readthedocs.io/en/latest/format.html>`_.
 
 Note that the method and variable names in PySpark are the similar case is ``threading`` library in Python itself where
 the APIs were inspired by Java. PySpark also follows `camelCase` for exposed APIs that match with Scala and Java.