Heartbeats guide: doc updates to reflect current behavior
Heartbeats guide: doc updates to reflect current behavior

See 
https://github.com/rabbitmq/rabbitmq-server/blob/33a7f97c4a471541adf05368d92862af4087c4a2/src/rabbit_reader.erl#L1211-L1213.

Closes #856.

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