mediawiki.user: Simplify implementation and jsduckify

Similar to mediawiki.util:
* Return plain object instead of an instantiated function.
* Refer to local reference instead of 'this' which makes it
  possible to use all methods "detached" without needing
  a specific context bound.
* Fix documentation comments be valid in jsduck syntax.
  - @example was invalid
  - "foo String : desc" was invalid, changed to:
    "{string} foo Desc"
  - type {false} was invalid, changed to {boolean}
  - reference mw.user.foo was invalid, changed to
    mw.user#foo.
  - getUserInfo was missing @private.
* Include in JSDuck index.

Change-Id: I2732cd7422f444b5cad1c6737a2de5dafb12d541
Merge "mediawiki.user: Simplify implementation and jsduckify"