Update heartbeat handling documentation

[DGames95]

mavlink_connection.wait_heartbeat() method does not error on timeout, it returns None mirroring recv_match()

The current documentation implies that the heartbeat is received as long as the method exits without error.

This updates the handling of a None type to avoid silently accepting a timeout.

[hamishwillee]

Thanks, but this is incorrect. By default hb = the_connection.wait_heartbeat() will either return with a connection or wait forever, so there is no case where this returns None. YOu could make it like that by adding a timeout, but that would be different code.