Improve the documentation of DBus identifiers

[poncovka]

Isolate the documentation for identifiers (https://github.com/rhinstaller/dasbus/blob/master/src/dasbus/identifier.py) and focus on using namespaces and functions from https://github.com/rhinstaller/dasbus/blob/master/src/dasbus/namespace.py. It is easier to grasp and good enough for smaller projects.