[SPARK-20132][DOCS] Add documentation for column string functions

## What changes were proposed in this pull request?
Add docstrings to column.py for the Column functions `rlike`, `like`, `startswith`, and `endswith`. Pass these docstrings through `_bin_op`

There may be a better place to put the docstrings. I put them immediately above the Column class.

## How was this patch tested?

I ran `make html` on my local computer to remake the documentation, and verified that the html pages were displaying the docstrings correctly. I tried running `dev-tests`, and the formatting tests passed. However, my mvn build didn't work I think due to issues on my computer.

These docstrings are my original work and free license.

davies has done the most recent work reorganizing `_bin_op`

Author: Michael Patterson <map222@gmail.com>

Closes #17469 from map222/patterson-documentation.