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

## What changes were proposed in this pull request?

With extract `[[` or replace `[[<-`, the parameter `i` is a column index, that needs to be corrected in doc. Also a few minor updates: examples, links.

## How was this patch tested?

manual

Author: Felix Cheung <felixcheung_m@hotmail.com>

Closes #16721 from felixcheung/rsubsetdoc.