JSDuck-ify /resources/mediawiki.language/*

* Removed some pieces of "documentation" I couldn't make any sense of. :)
* Marked some methods which looked internal as @private.
* Empirically tested and added some @return value types.
* Fixed format of doc comments to conform to JSDuck standards.
* Added entries to categories.json and config.json.

Also fixed jqueryMsg tests which depended on implementation details
and overwriting singleton classes and thus started failing now.
I have no idea what these hacks were for since the tests pass after
just removing them.

Change-Id: Ie60f72a5f277d846c09226d5af3da16b07f038c3
Merge "JSDuck-ify /resources/mediawiki.language/*"