First pass at documenting the WP_Filesystem methods. This also introduces stubs of the methods into the base class which are documented, which subclasses can override, some methods were cleaned up at the same time.
See #18476 See #23122. Props kurtpayne, bananastalktome, and, DrewAPicture 


git-svn-id: https://develop.svn.wordpress.org/trunk@25560 602fd350-edb4-49c9-b593-d223f7449a82