Update InputUnion.md

Author: leebyron

rfcs/InputUnion.md

@@ -161,6 +161,7 @@ If a solution places any restrictions on member types, compliance with these res
 In addition to containing Input types, member type may also contain Leaf types like `Scalar`s or `Enum`s.
 
 * Objection: Multiple Leaf types serialize the same way, making it impossible to distinguish the type. For example, a `String`, `ID` and `Enum`.
+  * Potential restrictions to fix: Limit to one leaf type in a union?
 * Objection: Output polymorphism is restricted to Object types only. Supporting Leaf types in Input polymorphism would create a new inconsistency.
 
 ## Use Cases