[SPARK-14393][SQL][DOC] update doc for python and R

## What changes were proposed in this pull request?

minor doc update that should go to master & branch-2.1

## How was this patch tested?

manual

Author: Felix Cheung <felixcheung_m@hotmail.com>

Closes #15747 from felixcheung/pySPARK-14393.

(cherry picked from commit a08463b1)
Signed-off-by: Felix Cheung <felixcheung@apache.org>