Update recommendation for registration to heartbeating

The recommendation to use registration predates having good support for STOMP
1.1 heartbeats in the stomp-gem and our connectors.

Here we rework the Note that outlines what registration does for us in terms of
keeping the connection alive and fault detection, as this also applies to
protocol heartbeats, and move it to the section on connector settings.
Merge pull request #229 from richardc/docs_heartbeat_not_registration

Update recommendation for registration to heartbeating