[SPARKR][DOCS] fix broken url in doc

## What changes were proposed in this pull request?

Fix broken url, also,

sparkR.session.stop doc page should have it in the header, instead of saying "sparkR.stop"
![image](https://cloud.githubusercontent.com/assets/8969467/17080129/26d41308-50d9-11e6-8967-79d6c920313f.png)

Data type section is in the middle of a list of gapply/gapplyCollect subsections:
![image](https://cloud.githubusercontent.com/assets/8969467/17080122/f992d00a-50d8-11e6-8f2c-fd5786213920.png)

## How was this patch tested?

manual test

Author: Felix Cheung <felixcheung_m@hotmail.com>

Closes #14329 from felixcheung/rdoclinkfix.