Document @return value for load_plugin_textdomain(). props jdgrimes.

Document $path parameter for load_child_theme_textdomain().

fixes #28690.
Built from https://develop.svn.wordpress.org/trunk@28924


git-svn-id: http://core.svn.wordpress.org/trunk@28723 1a063a9b-81f0-0310-95a4-ce76da25c4cd