Document jQuery.ready.promise()

Also:
* Categorise jQuery.holdReady in events/document-loading,
  to match ready() and jQuery.ready.promise().
* Categorise jQuery.holdReady in properties/global-jquery-object-properties,
  to match jQuery.fx.off() and jQuery.ready.promise().

Fixes #205
I understand it isn't a standards Promise (the definition of that seems still in flux, too), I just tried to match our terminology from elsewhere. What should this be called?

* `deferred.promise`: _"type=Promise", "Return a Deferred's Promise object.", "The Promise exposes only the Deferred methods needed to attach additional handlers or determine the state"_
* Types: _"Promise Object", "a subset of the methods of the Deferred object"_
* `promise`: _"a Promise object to observe when all [actions] have finished"_