[SPARK-16508][SPARKR] doc updates and more CRAN check fixes

## What changes were proposed in this pull request?

replace ``` ` ``` in code doc with `\code{thing}`
remove added `...` for drop(DataFrame)
fix remaining CRAN check warnings

## How was this patch tested?

create doc with knitr

junyangq

Author: Felix Cheung <felixcheung_m@hotmail.com>

Closes #14734 from felixcheung/rdoccleanup.