docs: clarify nothing special required for post-deployment migrations

doc/update/README.md

@@ -41,7 +41,8 @@ However, for this to work there are the following requirements:
 1. You can only upgrade 1 minor release at a time. So from 9.1 to 9.2, not to
    9.3.
 2. You have to use [post-deployment
-   migrations](../development/post_deployment_migrations.md).
+   migrations](../development/post_deployment_migrations.md) (included in
+   zero downtime update steps below)
 3. You are using PostgreSQL. If you are using MySQL please look at the release
    post to see if downtime is required.