[Doc] Improve Python DataFrame documentation

Author: Reynold Xin <rxin@databricks.com>

Closes #5287 from rxin/pyspark-df-doc-cleanup-context and squashes the following commits:

1841b60 [Reynold Xin] Lint.
f2007f1 [Reynold Xin] functions and types.
bc3b72b [Reynold Xin] More improvements to DataFrame Python doc.
ac1d4c0 [Reynold Xin] Bug fix.
b163365 [Reynold Xin] Python fix. Added Experimental flag to DataFrameNaFunctions.
608422d [Reynold Xin] [Doc] Cleanup context.py Python docs.

(cherry picked from commit 305abe1e)
Signed-off-by: Reynold Xin <rxin@databricks.com>