[SPARKR][BRANCH-2.0] R merge API doc and example fix

## What changes were proposed in this pull request?

Fixes for R doc

## How was this patch tested?

N/A

Author: Felix Cheung <felixcheung_m@hotmail.com>

Closes #15589 from felixcheung/rdocmergefix.

(cherry picked from commit 0e0d83a5)
Signed-off-by: Felix Cheung <felixcheung@apache.org>