Document jQuery.ready

[Krinkle]

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

[Krinkle]

(Replying to now-closed pull #530)

@dmethvin Should this be documented as jQuery.ready or jQuery.ready.then? I assume the former. However we don't currently have an entry at Types for a pure "thenable". I.e. something that is not itself a $.Deferred, $.Deferred#promise or native Promise, but only returns one after calling .then().

[dmethvin]

@Krinkle Yep, it's jQuery.ready and I think we need to create a Thenable type.

[Krinkle]

Blocked on #906. (Add Thenable type, and update jQuery.when as receiving it in addition to Promise/jQuery.Deferred)

[Krinkle]

Unblocked! Rebased for #1026 (landed).

@dmethvin Could you re-review?

[dmethvin]

This looks good, we need to mark holdReady as deprecated but there's already a ticket for that in #972 . Thanks!