doc(intersection): mention non-variant

Author: robstoll

docs/docs/reference/new-types/intersection-types-spec.md

@@ -45,11 +45,11 @@ A & B <: B       A & B <: A
 In another word, `A & B` is the same type as `B & A`, in the sense that the two types
 have the same values and are subtypes of each other.
 
-If `C` is a type constructor, the join `C[A] & C[B]` is simplified by pulling the
-intersection inside the constructor, using the following two rules:
+If `C` is a type constructor, then `C[A] & C[B]` can be simplified using the following three rules:
 
 - If `C` is covariant, `C[A] & C[B] ~> C[A & B]`
 - If `C` is contravariant, `C[A] & C[B] ~> C[A | B]`
+- If `C` is non-variant, emit a compile error
 
 When `C` is covariant, `C[A & B] <: C[A] & C[B]` can be derived: