mediawiki.Title: Fix typo "add" in documentation
Follows-up
3d14628237.
* Put @class after the description for consistency with other
classes and methods.
* Remove jscs:disable rule which was there for the illegal
@constructor use (descriptions must go before a tag, not after).
However JSDuck doesn't support descriptions before @constructor.
It supports it with all other tags, however.
It turns out, that @constructor doesn't support this because
it is deprecated and predates that feature in JSDuck. Upstream
documentation encourages authors to use @method instead.
Change-Id: Iba9fa5ae1fc8ece9ddff4aef8a3b4b59768fcaf0