Doc update: Clarify got_request_exception signal docs

[tonydelanuez]

got_request_exception documentation did not explain that the signal is only sent for unhandled exceptions, not HTTPException or other handled exceptions.

closes #3851

[davidism]

The existing section was already bit confusing, so I did a further rewrite based on your changes. It describes that it's sent for unhandled exceptions, and not sent for HTTPExcpetion and handled exceptions. Also rewrote the example to be more interesting, since exceptions are already logged anyway.