Add when migrations are run on GitLab deploys

This question comes often when reviewing MRs or on Slack and it's not
documenting anywhere, and it should be considered when adding regular or
post migrations

doc/development/migration_style_guide.md

@@ -15,8 +15,8 @@ offline unless _absolutely necessary_.
 When downtime is necessary the migration has to be approved by:
 
 1. The VP of Engineering
-1. A Backend Lead
-1. A Database Specialist
+1. A Backend Maintainer
+1. A Database Maintainer
 
 An up-to-date list of people holding these titles can be found at
 <https://about.gitlab.com/company/team/>.
@@ -29,6 +29,10 @@ Please don't depend on GitLab-specific code since it can change in future
 versions. If needed copy-paste GitLab code into the migration to make it forward
 compatible.
 
+For GitLab.com, please take into consideration that regular migrations (under `db/migrate`)
+are run before [Canary is deployed](https://about.gitlab.com/handbook/engineering/infrastructure/library/canary/#configuration-and-deployment),
+and post-deployment migrations (`db/post_migrate`) are run after the deployment to production has finished.
+
 ## Schema Changes
 
 Migrations that make changes to the database schema (e.g. adding a column) can