Deprecate ajax "global" event behavior

[dmethvin]

Currently the ajax events are "global" events in that they will fire on any element, e.g., $(".anything").ajaxStart(fn). The element doesn't even need to be attached to the DOM! This requires us to do some expensive bookkeeping and special-case code.

As of jQuery 1.8, the docs should encourage users to attach ajax events only to document. In 1.9 we can have them only fire on document, with a plugin for backcompat.

[dmethvin]

Although the ajax events are the only ones in the public API to be global events, the event subsystem allows any caller to trigger a global event by passing a null to jQuery.event.trigger (which isn't a public API). So this is deprecating both some ajax behavior and an internal event feature.