doc: Improve documentation for mw.Map and mw#config

* Link to the list on mediawiki.org
* Document it as type mw.Map instead of implied from code (= null)
  the fact that the literal code in mediawiki.js is a placeholder
  is an implementation detail that shouldn't be in the API docs.
* Add example for mw.Map.

Change-Id: I5c3ff272e605ee3825320f27aec32a4b4bf18ee2
Merge "doc: Improve documentation for mw.Map and mw#config"