Add getId to mw.user, returning wgUserId (or 0 for anons).

This is equivalent to how User->getId works on the server.

Small formatting update to another documentation block.

Change-Id: I31906398f572dd4f62d89b53dfd05a362bf6d5a5
Merge "Add getId to mw.user, returning wgUserId (or 0 for anons)."