Add description on test

Author: spastorino

src/test/ui/associated-type-bounds/super-trait-referencing.rs

@@ -1,4 +1,8 @@
 // check-pass
+
+// The goal of this test is to ensure that T: Bar<T::Item>
+// in the where clause does not cycle
+
 trait Foo {
     type Item;
 }