Add clarification at C.21 (#1792)

CppCoreGuidelines.md

@@ -4745,7 +4745,7 @@ into more expensive copies, or making a class move-only.
         // ...
     }
 
-Given that "special attention" was needed for the destructor (here, to deallocate), the likelihood that copy and move assignment (both will implicitly destroy an object) are correct is low (here, we would get double deletion).
+Given that "special attention" was needed for the destructor (here, to deallocate), the likelihood that copy and move assignment (which are generated by the compiler and both will implicitly destroy an object) are correct is low (here, we would get double deletion).
 
 ##### Note