Improved documentation and handling of config classes

Did some minor improvements to the doc strings.

I also noted that if type annotation isn't added to the NomadConfig classes, the attributes might be listed in a strange order in the generated documentation, different from the order in which the attributes are defined, so I added some type annotations to make sure the logical order is preserved.

I also took myself the liberty to change the behaviour of the customize method so that it can take either a NomadConfig object or a dictionary as its sole argument (this meant I could simplify the code in some places, and I think it is cleaner. The important thing is that the dict is converted to an instance of the right NomadConfig class before we start accessing various settings, and I think this should be the responsibility of the customize method. This method also does all the checks in one place.) There was also a bug in the CLI commands relating to this (fixed now).