message: core: improve connection docstring
  The behavior of add_any_connection had changed in:
diff: @@ -72,9 +72,14 @@ public:
  *
  * Supports connection: Serial, TCP or UDP.
  * Connection URL format should be:
  - * - UDP - udp://[Bind_host][:Bind_port]
  - * - TCP - tcp://[Remote_host][:Remote_port]
  - * - Serial - serial://Dev_Node[:Baudrate]
  + * - UDP: udp://[host][:bind_port]
  + * - TCP: tcp://[host][:remote_port]
  + * - Serial: serial://dev_node[:baudrate]
  + *
  + * For UDP, the host can be set to either:
  + * - zero IP: 0.0.0.0 -> behave like a server and listen for heartbeats.
  + * - some IP: 192.168.1.12 -> behave like a client, initiate connection
  + * and start sending heartbeats.
  *
  * @param connection_url connection URL string.
  * @param forwarding_option message forwarding option (when multiple interfaces are used).
  