libnm: don't skip NMObject:path from documentation and introspection
The D-Bus path is a useful property, also exposed via nm_object_get_path() function. Don't mark it to be skipped from documentation and introspection.
This was changed in "1f5b48a5 libnm: use the o.fd.DBus.ObjectManager API for object management" and was an API breakage of libnm. This also caused a bug in gnome-shell [1].
[1] https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/240/diffs [2] https://bugzilla.redhat.com/show_bug.cgi?id=1628263
Fixes: 1f5b48a5