document that Specifying the url, data, and dataType parameter without specifying the callback parameter breaks $.ajax

[mikesherov]

from @dmethvin:

I think we should document that the callback must be there, even if it's just a null placeholder. That way the string following it is always the dataType and can't be mistaken for the data arg. Does that seem reasonable? If so add a needsdocs.

ported from http://bugs.jquery.com/ticket/12233