[SPARK-15863][SQL][DOC][SPARKR] sql programming guide updates to include sparkSession in R

## What changes were proposed in this pull request?

Update doc as per discussion in PR #13592

## How was this patch tested?

manual

shivaram liancheng

Author: Felix Cheung <felixcheung_m@hotmail.com>

Closes #13799 from felixcheung/rsqlprogrammingguide.