Better documentation for MessageQueueHandler

I've actually removed docs for RedisHandler as `MessageQueueHandler` already supports it. Included documentation for RabbitMQ, Redis and MongoDB as those 3 seem to be the ones properly supported by `kombu`. (Mysterious footnote regarding the rest of them: http://kombu.readthedocs.org/en/latest/introduction.html#f1 )

Requires proof reading.

Removed an unused parameter as it seemed like no one would miss it.