Document .end class for grids

Documentation for the `end` class in grids has been missing since Foundation 4, even though the class still exists and is very useful.

I added documentation and an example for the class, which are mostly copied from [Foundation 3's documentation](http://foundation.zurb.com/docs/v/3.2.5/grid.php).

![screen shot 2014-01-21 at 12 06 07 pm](https://f.cloud.github.com/assets/1009531/1967833/ccb87b72-82d9-11e3-9a7b-61b912b6d853.png)

See #3660 and #4175 for evidence that documentation is needed.