Address review comments.

working/macros/feature-specification.md

@@ -273,7 +273,9 @@ whatever the application order. In most cases this achieved by the split into
 phases: within each phase macros can run in any order because the output is not
 visible to other macros until the next phase. As a special case, introspection
 of types in Phase 2 waits as needed for other macro applications to complete,
-failing if there are cycles.
+failing if there is a cycle.
+
+TODO: give an example of a cycle.
 
 ### Augmentation library structure and ordering