[SPARK-15319][SPARKR][DOCS] Fix SparkR doc layout for corr and other DataFrame stats functions

## What changes were proposed in this pull request?

Doc only changes. Please see screenshots.

Before:
http://spark.apache.org/docs/latest/api/R/statfunctions.html
![image](https://cloud.githubusercontent.com/assets/8969467/15264110/cd458826-1924-11e6-85bd-8ee2e2e1a85f.png)

After
![image](https://cloud.githubusercontent.com/assets/8969467/16218452/b9e89f08-3732-11e6-969d-a3a1796e7ad0.png)
(please ignore the style differences - this is due to not having the css in my local copy)

This is still a bit weird. As discussed in SPARK-15237, I think the better approach is to separate out the DataFrame stats function instead of putting everything on one page. At least now it is clearer which description is on which function.

## How was this patch tested?

Build doc

Author: Felix Cheung <felixcheung_m@hotmail.com>
Author: felixcheung <felixcheung_m@hotmail.com>

Closes #13109 from felixcheung/rstatdoc.