Consumers guide: explain how message type and content {type, encoding} properties should be used
Consumers guide: explain how message type and content {type, encoding} properties should be used

Reviewer: da39a3ee5e6b4b0d3255bfef95601890afd80709@lukebakken.

rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website