Update an outdated test explanation

Author: oli-obk

src/test/ui/type-alias-impl-trait/fallback.rs

@@ -1,5 +1,5 @@
-// Tests that we correctly handle the instantiated
-// inference variable being completely unconstrained.
+// Tests that we correctly handle opaque types being used opaquely,
+// even within their defining scope.
 //
 // check-pass
 #![feature(type_alias_impl_trait)]