Add decsription of Eq desugarings for enums

Author: odersky

docs/docs/reference/desugarEnums.md

@@ -138,6 +138,15 @@ comma separated simple cases into a sequence of cases.
    Any modifiers or annotations on the original case extend to all expanded
    cases.
 
+## Equality
+
+An `enum` type contains a `scala.Eq` instance that restricts values of the `enum` type to
+be compared only to other values of the same enum type. Furtermore, generic
+`enum` types are comparable only if their type arguments are. For instance the
+`Option` enum type will get the following definition in its companion object:
+
+    implicit def eqOption[T, U](implicit ev1: Eq[T, U]): Eq[Option[T], Option[U]] = Eq
+
 ## Translation of Enumerations
 
 Non-generic enum classes `E` that define one or more singleton cases