mediawiki.user: Clean up documentation and minor optimisation

mediawiki.user: Clean up documentation and minor optimisation

* Optimise mw.user#id implementation.
* Make documentation more consistent (describe the intent instead
  of the exhibited behaviour, e.g. "Get foo" instead of "Gets foo",
  like most other JS documentation does already).

Change-Id: If9e5dcb7cfed27785c4a15bc3492868f083517f3