[SPARKR][MINOR] Fix docs for sparkR.session and count

## What changes were proposed in this pull request?

This PR tries to add some more explanation to `sparkR.session`. It also modifies doc for `count` so when grouped in one doc, the description doesn't confuse users.

## How was this patch tested?

Manual test.

![screen shot 2016-09-02 at 1 21 36 pm](https://cloud.githubusercontent.com/assets/15318264/18217198/409613ac-7110-11e6-8dae-cb0c8df557bf.png)

Author: Junyang Qian <junyangq@databricks.com>

Closes #14942 from junyangq/fixSparkRSessionDoc.