Expand the Ajax Convenience Methods to talk about jqXHR and chainable error handling

[RedWolves]

Separating out the second issue from issue #555 so a partial fix can be applied.

The first phrase in the Convenience Methods-section, however, says this:

If you don't need the extensive configurability of $.ajax(), and you don't care about handling errors... Handling the errors point is kinda misleading, as of jQuery 1.5. all jQuery Ajax-methods return a jqXHR-object, which is chainable with .error(), .done() and .always() (or as of 1.8 with .fail(), .done() and .always()), which makes using the convenience methods while handling error situations possible.