Fill out inline documentation for magic methods added to the `WP_Filesystem_Base` class in [28521].

See #22234 and #28885.

Built from https://develop.svn.wordpress.org/trunk@29147


git-svn-id: http://core.svn.wordpress.org/trunk@28931 1a063a9b-81f0-0310-95a4-ce76da25c4cd