Document MqttUtf8String.toString() behavior

[Marcono1234]

Problem or use case

It appears the intention of MqttUtf8String.toString() is to return the decoded String (see also implementation). However, this is currently not documented; toString() is not overridden and its documentation is not adjusted. It is therefore not obvious how a user can easily get a String from a MqttUtf8String.

Preferred solution or suggestions

If MqttUtf8String.toString() is guaranteed to return the decoded String, then MqttUtf8String should override toString() and add a Javadoc comment to it describing its behavior.