Update CppCoreGuidelines.md

Clarified the enforcement of C.121 according to #1368

CppCoreGuidelines.md

@@ -6926,7 +6926,7 @@ Give `Goof` a virtual destructor and all is well.
 
 ##### Enforcement
 
-* Warn on any class that contains data members and also has an overridable (non-`final`) virtual function.
+* Warn on any class that contains data members and also has an overridable (non-`final`) virtual function tht wasn't inherited from a base class.
 
 ### <a name="Rh-separation"></a>C.122: Use abstract classes as interfaces when complete separation of interface and implementation is needed