Add the test description

Author: tiif

tests/ui/internal/unstable-impl-assoc-type.rs

@@ -6,7 +6,7 @@
 #![feature(impl_stability)]
 #![unstable(feature = "feat_foo", issue = "none" )]
 
-/// FIXME: add a description here.
+/// Test that you can't leak unstable impls through item bounds on associated types.
 
 trait Bar {}