Clarify/correct inline documentation in wp-includes/l10n.php.

* Add a missing `@return` description for `load_default_textdomain()`
* Clarify the `@param` description for `$locale` with an inline `@see` tag
* Ensure the correct parameter variable is used when documenting `$args` for `wp_dropdown_languages()`

Props camdensegal.
Fixes #30385.


git-svn-id: https://develop.svn.wordpress.org/trunk@30380 602fd350-edb4-49c9-b593-d223f7449a82