Add JSDoc for JQueryXHR.abort method.

Inspired the description from its source code:
https://github.com/jquery/jquery/blob/d41e81980ea89e56c04eb14d8c27f4692b367604/src/ajax.js#L487
Merge pull request #2316 from lijunle/jquery-xhr-doc

Add JSDoc for JQueryXHR.abort method.