Specify precisely where it is an error to declare an initializing formal (#3517)

Specify precisely where it is an error to declare an initializing formal parameter (this used to be commentary, should be normative).

Co-authored-by: Nate Bosch <[email protected]>

Author: eernstg

specification/dartLangSpec.tex

@@ -47,6 +47,8 @@
 % - Broaden the grammar rule about `initializerExpression` to match the
 %   implemented behavior. Specify that an initializer expression can not be
 %   a function literal.
+% - Specify in which situations it is an error to declare an initializing
+%   formal parameter.
 %
 % Nov 2023
 % - Specify that the dynamic error for calling a function in a deferred and
@@ -4003,12 +4005,12 @@ \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 or enum.
 
-\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 or enum.%
-}
+\LMHash{}%
+It is a compile-time error for an initializing formal parameter
+to occur in any function which is not a generative constructor.
+Also, it is a compile-time error for an initializing formal parameter
+to occur in a redirecting or external constructor.
+\commentary{In particuar, there is always an enclosing class or enum.}
 
 \LMHash{}%
 Assume that $p$ is a declaration of an initializing formal parameter named \id.