Add note clarifying a confusing method call

The call is made via user-supplied string, so a code search
will not find where the method is used.
Adding a note to help prevent future confusion.

Change-Id: Ib1fc34d0cc9ca5ee54111c8ffe43db30c8a48d96

congress/api/policy_model.py

@@ -179,6 +179,9 @@ class PolicyModel(base.APIModel):
         value = params[key]
         return value.lower() == "true" or value == "1"
 
+    # Note: It's confusing to figure out how this method is called.
+    # It is called via user supplied string in the `action` method of
+    # api/webservice.py:ElementHandler
     # Note(thread-safety): blocking function
     def simulate_action(self, params, context=None, request=None):
         """Simulate the effects of executing a sequence of updates.