Removing incorrect jQuery XHR example from $.when() docs. Adding documentation on the arguments passed to done/ fail callbacks.

Rationale for removing the XHR example rather than correcting it is that an equivalent example is also provided at the end of the documentation, and I struggled to fit the example into the new form, which includes documentation on the values passed to the master-Deferred doneCallback, which was previously undocumented.

This would complete #262 (or otherwise make it redundant).