Make adjustments following issue #4015 (#4103)

Introduce compile-time errors about augmentation of "built-in" properties of an `enum` declaration, following issue #4015.

Author: eernstg

working/augmentation-libraries/feature-specification.md

@@ -726,7 +726,15 @@ It is a **compile-time error** if:
 
 *   An `external` variable is augmented with an `abstract` variable.
 
-### Augmenting enum values
+### Augmenting enum members
+
+Some enum members can not be augmented: It is a compile-time error if an
+augmenting declaration in an enum declaration (introductory or augmenting)
+has the name `values`, `index`, `hashCode`, or `==`.
+
+*It has always been an error for an enum declaration to declare a member
+named `index`, `hashCode`, `==`, or `values`, and this rule just clarifies
+that this error is applicable for augmenting declarations as well.*
 
 Enum values can _only_ be augmented by enum values, and the implicit getter
 introduced by them is not augmentable. The only thing you are allowed to do