Fix #6601: Require that enum cases extend the enum

Author: odersky

docs/docs/internals/syntax.md

@@ -105,7 +105,7 @@ yield
 
 ```
 as        derives   extension inline    on        opaque    open      using
-~         *         |         &         +         -
+*         +         -
 ```
 
 ## Context-free Syntax

docs/docs/reference/enums/desugarEnums.md

@@ -196,6 +196,9 @@ Cases such as `case C` expand to a `@static val` as opposed to a `val`. This all
 
 ### Other Rules
 
-A normal case class which is not produced from an enum case is not allowed to extend
+ - A normal case class which is not produced from an enum case is not allowed to extend
 `scala.Enum`. This ensures that the only cases of an enum are the ones that are
 explicitly declared in it.
+
+ - If an enum case has an extends clause, the enum class must be one of the
+   classes that's extended.