[jQuery.ajax] Document callback order

[timmywil]

Ported form http://bugs.jquery.com/ticket/14011

[dmethvin]

@timmywil did you have some specific ideas on what to document here? I am thinking that specifying the exact order of all things ajaxy may overconstrain us. There are some specifics that could be helpful (success runs before complete) but that is already documented. IMO we're good already.

[scottgonzalez]

From the ticket, it looks like the request is to document when statusCode callbacks are executed in relation to the promise callbacks.