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
In the standards sense, it doesn't provide a Promise interface but instead a thenable one. This has been one of the reasons we've been hesitant to document this. The other is that we may try to make jQuery.Deferred optional in a future version, so it's almost as if we're documenting and deprecating this at the same time.