Improve mediawiki.js documentation

Improve mediawiki.js documentation

Incorporate some of the edits made to the old documentation on
mediawiki.org at https://www.mediawiki.org/wiki/ResourceLoader/Default_modules

mw:
* Add note about the globals being added.

mw.Message:
* Add inline example.
* Revise documentation of related methods.

mw.loader:
* Add code samples for #addSource and #using.
* Document loadScript property for #addSource using @param syntax.
* Document "null" return value for #getVersion.

mw.html:
* Add code sample #escape.

mw.loader.store:
* Mark #update as method instead of the incorrectly inferred
  type "property" (the assignment is non-trivial, it can't
  infer the return type of the closure).

Change-Id: Id42a108066fac333a9253f2fa8908746b8a1ba8f