Clarify that an extension type _can_ have members which are external instance variables (#3303)

Author: eernstg

accepted/future-releases/extension-types/feature-specification.md

@@ -429,7 +429,10 @@ disambiguate the extension type declaration with a fixed lookahead.*
 Some errors can be detected immediately from the syntax:
 
 A compile-time error occurs if the extension type declaration declares any
-instance variables.
+instance variables, unless they are `external`.
+
+*An external instance variable is just a convenient notation for an external
+getter and (if not `final`) an external setter. They are allowed.*
 
 The _name of the representation_ in an extension type declaration with a
 representation declaration of the form `(T id)` is the identifier `id`, and