Specify precisely where it is an error to declare an initializing for…

…mal parameter

specification/dartLangSpec.tex

@@ -3652,12 +3652,12 @@ \subsubsection{Generative Constructors}
 It is a compile-time error if \id{} is not also the name of
 an instance variable of the immediately enclosing class.
 
-\commentary{%
-Note that it is a compile-time error for an initializing formal
-to occur in any function which is not a non-redirecting generative constructor
-(\ref{requiredFormals}),
-so there is always an enclosing class.%
-}
+\LMHash{}%
+It is a compile-time error for an initializing formal parameter
+to occur in any function which is not
+a non-redirecting, non-external, generative constructor
+(\ref{requiredFormals}).
+\commentary{This implies that there is always an enclosing class. }
 
 \LMHash{}%
 Assume that $p$ is a declaration of an initializing formal parameter named \id.