Remove redundant table alias check in WP_Meta_Query.

Also adds documentation for 'meta_query_find_compatible_table_alias' filter.

See #24093.
Built from https://develop.svn.wordpress.org/trunk@29953


git-svn-id: http://core.svn.wordpress.org/trunk@29701 1a063a9b-81f0-0310-95a4-ce76da25c4cd