api: mark mandatory, default, optional_packages as api and add api doc

dnf.comps.Groups are not very usefull, if there is no way to get the packages they contains
so i have marked mandatory, default, optional_packages as api.

If you like i can change the code to return a list of package names, instead of the undocumented libcomps.package pbjects, but that will need some changes in the cli too :)