[BACKPORT-2.1][SPARKR][DOCS] update R API doc for subset/extract

## What changes were proposed in this pull request?

backport #16721 to branch-2.1

## How was this patch tested?

manual

Author: Felix Cheung <felixcheung_m@hotmail.com>

Closes #16749 from felixcheung/rsubsetdocbackport.