[SPARKR][DOCS] R code doc cleanup

## What changes were proposed in this pull request?

I ran a full pass from A to Z and fixed the obvious duplications, improper grouping etc.

There are still more doc issues to be cleaned up.

## How was this patch tested?

manual tests

Author: Felix Cheung <felixcheung_m@hotmail.com>

Closes #13798 from felixcheung/rdocseealso.