Ajax: add note on how to bypass query string alteration for JSON requests

[timmywil]

See jquery/jquery#1799

JSON requests will sometimes get promoted to JSONP if deemed cross-browser (we already document that part). We add a callback name to the query string for JSONP requests (we document that too). The part that isn't documented is that in rare cases, the data for a json request contains a question mark, and so the callback name may be injected into a weird part of the string. Users can work around this by setting jsonp to false.