Documentation mqtt client id

[hannes-angst]

== This is just a reminder to update documentation ==

To login to a mqtt server, you provide a client id, user name, and password.

The client id needs to be unique. If not, clients using the same client id will login in, log out, forever.

I learned that the hard way ;(

Tested: mosquitto mqtt server.

 char *clientId = (char*) os_zalloc(64);
 os_sprintf(clientId, "%s%08X", MQTT_CLIENT_ID, system_get_chip_id());
 MQTT_InitClient(&mqttClient, clientId, user, pass, MQTT_KEEPALIVE, MQTT_CLEAN_SESSION);
 os_free(clientId);