add clarifying comment

lib/spack/spack/repo.py

@@ -1366,6 +1366,10 @@ def get_pkg_class(self, pkg_name):
         overidden_attrs = getattr(cls, 'overidden_attrs', {})
         for key, val in overidden_attrs.items():
             setattr(cls, key, val)
+        # Keep track of every class attribute that is overidden by the config:
+        # if the config changes between calls to this method, we make sure to
+        # restore the original config values (in case the new config no longer
+        # sets attributes that it used to)
         new_overidden_attrs = {}
         for key, val in new_cfg_settings.items():
             new_overidden_attrs[key] = getattr(cls, key)