Updates release post info on background migration

Background migrations used to be announced on the barometer section on
every release post. This section was deprecated several releases ago and
replaced by an Upgrade section that only announces changes for a major
update.

doc/development/background_migrations.md

@@ -10,11 +10,6 @@ migrations automatically reschedule themselves for a later point in time.
 
 ## When To Use Background Migrations
 
-> **Note:**
-> When adding background migrations _you must_ make sure they are announced in the
-> monthly release post along with an estimate of how long it will take to complete
-> the migrations.
-
 In the vast majority of cases you will want to use a regular Rails migration
 instead. Background migrations should be used when migrating _data_ in
 tables that have so many rows this process would take hours when performed in a
@@ -34,6 +29,11 @@ Some examples where background migrations can be useful:
 - Populating one column based on JSON stored in another column.
 - Migrating data that depends on the output of external services (e.g. an API).
 
+> **Note:**
+> If the background migration is part of an important upgrade, make sure it's announced
+> in the release post. Discuss with your Project Manager if you're not sure the migration falls
+> into this category.
+
 ## Isolation
 
 Background migrations must be isolated and can not use application code (e.g.