ajax: Document timeout 0 as meaning no timeout

[Krinkle]

It seems this has been in the code for a long time, but was never officially documented. It'd be nice to know that is officially supported and not merely an implementation detail.

http://api.jquery.com/jQuery.ajax/

If not, we should document another value as meaning no timeout (e.g. false, null or something else). This is mainly applicable to use cases where $.ajax() is used inside an abstraction layer that provides some default options (using $.extend or something) so that the user has a way to disable the default timeout.