message format in user configuration is undocumented

Summary

While inspecting #509 (closed), we noticed that the message-format configuration is not properly documented.

We need to explain how this works in the user configuration docs, and ensure that we've listed and explained all of the possible identifiers which are valid for the message format.

The identifiers can be gleaned from the source currently at: https://gitlab.com/BuildStream/buildstream/blob/master/buildstream/_frontend/widget.py#L328