Added the missing documentation for the correlation tracker.
Didn't realise that the tracker returned the side lobe ratio,
so fixed the API to return it properly.

This aids an API for the correlation based tracker that Davis
recently added to Dlib. I've made sure to allow overrides for
passing normal rectangles rather than drectangles (which isn't
currently supported in the Dlib C++ API). This is mostly
because I imagine people might initialize the tracking using
something like a bounding box from a detector (e.g.
load_frontal_face_detector).