4.1 Docs Audit: Improve inline documentation for various `WP_Meta_Query` methods introduced or modified in 4.1.

See #30469.

Built from https://develop.svn.wordpress.org/trunk@30617


git-svn-id: http://core.svn.wordpress.org/trunk@30607 1a063a9b-81f0-0310-95a4-ce76da25c4cd