This closes #3412.

The documentation on AFHTTPSessionManager, Methods to Override section states that...

To change the behavior of all data task operation construction, which is also used in the GET / POST / et al. convenience methods, override dataTaskWithRequest:completionHandler:

But that is no longer accurate, since these convenience methods now go through the new
dataTaskWithRequest:uploadProgress:downloadProgress:completionHandler:. Therefore, overriding dataTaskWithRequest:completionHandler: will have no effect since it will never be called.

This PR updates the documentation to reflect the fact that the method to override is now dataTaskWithRequest:uploadProgress:downloadProgress:completionHandler:.