Document that production waits for the staging revision clean-up

The prod deployment needs the whole beta deployment, which now includes
the revision clean-up that runs after a staging promotion, so a failed
staging clean-up also stops the production deployment.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: anth-volk

README.md

@@ -97,7 +97,8 @@ time:
 6. verify the stable endpoint, restoring its previous revision if immediate
    post-promotion checks fail; and
 7. begin the `prod` deployment only after every `beta` deployment,
-   qualification and promotion job succeeds.
+   qualification, promotion and revision clean-up job succeeds, so a failed
+   staging clean-up also stops the production deployment.
 
 The staging service's stable URL therefore serves the revision that passed the
 staging tests, so staging callers of that URL, such as the Public API's staging