[SPARK-16508][SPARKR] Split docs for arrange and orderBy methods

## What changes were proposed in this pull request?

This PR splits arrange and orderBy methods according to their functionality (the former for sorting sparkDataFrame and the latter for windowSpec).

## How was this patch tested?

![screen shot 2016-08-06 at 6 39 19 pm](https://cloud.githubusercontent.com/assets/15318264/17459969/51eade28-5c05-11e6-8ca1-8d8a8e344bab.png)
![screen shot 2016-08-06 at 6 39 29 pm](https://cloud.githubusercontent.com/assets/15318264/17459966/51e3c246-5c05-11e6-8d35-3e905ca48676.png)
![screen shot 2016-08-06 at 6 40 02 pm](https://cloud.githubusercontent.com/assets/15318264/17459967/51e650ec-5c05-11e6-8698-0f037f5199ff.png)

Author: Junyang Qian <junyangq@databricks.com>

Closes #14522 from junyangq/SPARK-16508-0.