gresolver: Document that GResolver lists are non-empty on success

The documentation previously wasn’t clear about whether the GResolver
methods could return an empty list and no error. On balance, this seems
like a bad idea, and GResolver should commit to always return a
non-empty list, or an error (which should be G_RESOLVER_ERROR_NOT_FOUND
if the list would otherwise be empty).

https://bugzilla.gnome.org/show_bug.cgi?id=728776