Inline documentation cleanup for 4.0 audit.

* phpDoc tweaks for `wp_get_available_translations_from_api()`, `wp_install_download_language_pack()`, and `wp_install_load_language()`, added in [29018]
 
See #28885.

Built from https://develop.svn.wordpress.org/trunk@29154


git-svn-id: http://core.svn.wordpress.org/trunk@28938 1a063a9b-81f0-0310-95a4-ce76da25c4cd