[SPARK-16096][SPARKR] add union and deprecate unionAll

## What changes were proposed in this pull request?

add union and deprecate unionAll, separate roxygen2 doc for rbind (since their usage and parameter lists are quite different)

`explode` is also deprecated - but seems like replacement is a combination of calls; not sure if we should deprecate it in SparkR, yet.

## How was this patch tested?

unit tests, manual checks for r doc

Author: Felix Cheung <felixcheung_m@hotmail.com>

Closes #13805 from felixcheung/runion.