add comments for forbidden structure check process

Author: KEHANG

rmgpy/data/kinetics/family.py

@@ -1331,8 +1331,11 @@ def isMoleculeForbidden(self, molecule):
 
         forbidden_structures = getDB('forbidden')
 
+        # check family-specific forbidden structures 
         if self.forbidden is not None and self.forbidden.isMoleculeForbidden(molecule):
             return True
+
+        # check RMG globally forbidden structures
         if forbidden_structures.isMoleculeForbidden(molecule):
             return True
         return False