Tweak login/account creation for logged in users

* Don't show the "Don't have an account? [Join <sitename>]" box to
  logged-in users.
* Use different message for "Real name" (disabled on WMF wikis) when
  creating another's account.

Also document showCreateOrLoginLink() and set its visibility.

Bug: 43172
Change-Id: I1b0bb6b78cbc5d37ef9f380a4bdf0f65d021a57f