mediawiki.js: Clean up and improve documentation

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