## 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.