mediawiki.js: Clean up and improve documentation
* Consistently phrase one-line captions in the imperative mood.
* Consistently start parameter descriptions with a capital letter.
* Document a few missing parameters and callback parameters.
* Rewrite mw.Map example.
* Merge duplicate @return tags in mw.Map#set (only the first one
is parsed by JSDuck).
* Fix mw.loader#registry code example. In order for the indented
text to be parsed as a code block, an empty is required.
* Improve IE-support comments with more details.
* Remove redundant String() cast in newStyleTag(). Per its signature,
it only supports strings. Other values don't make sense.
Change-Id: Iaaeaea4869f7869f5f4d29b27d11c0b05aad2ef5