Explain what a type is on messages

[grimurd]

The documentation should include what a message type means in rabbot since it's not a part of amqp.

Only explanation i've found was was written by @derickbailey here

This should be a part of the documentation somewhere

[et]

This tripped me up too. I have no clue why I need this. I use the basic amqplib and publishing with a routing key seems sufficient. I don't know why there needs to be yet another piece of terminology.