Improvements to migrations docs

doc/administration/raketasks/maintenance.md

@@ -255,7 +255,10 @@ sudo gitlab-rake gitlab:exclusive_lease:clear[project_housekeeping:4]
 
 ## Display status of database migrations
 
-To check the status of migrations, you can use the following rake task:
+See the [upgrade documentation](../../update/README.md#checking-for-background-migrations-before-upgrading)
+for how to check that migrations are complete when upgrading GitLab.
+
+To check the status of specific migrations, you can use the following rake task:
 
 ```shell
 sudo gitlab-rake db:migrate:status

doc/update/README.md

@@ -122,13 +122,15 @@ If using GitLab 12.9 and newer, run:
 sudo gitlab-rails runner -e production 'puts Gitlab::BackgroundMigration.remaining'
 ```
 
-If using GitLab 12.8 and older, run the following using a Rails console:
+If using GitLab 12.8 and older, run the following using a [Rails console](../administration/troubleshooting/debug.md#starting-a-rails-console):
 
 ```ruby
 puts Sidekiq::Queue.new("background_migration").size
 Sidekiq::ScheduledSet.new.select { |r| r.klass == 'BackgroundMigrationWorker' }.size
 ```
 
+---
+
 **For installations from source**
 
 If using GitLab 12.9 and newer, run:
@@ -138,13 +140,16 @@ cd /home/git/gitlab
 sudo -u git -H bundle exec rails runner -e production 'puts Gitlab::BackgroundMigration.remaining'
 ```
 
-If using GitLab 12.8 and older, run the following using a Rails console:
+If using GitLab 12.8 and older, run the following using a [Rails console](../administration/troubleshooting/debug.md#starting-a-rails-console):
 
 ```ruby
 puts Sidekiq::Queue.new("background_migration").size
 Sidekiq::ScheduledSet.new.select { |r| r.klass == 'BackgroundMigrationWorker' }.size
 ```
 
+There is also a [rake task](../administration/raketasks/maintenance.md#display-status-of-database-migrations)
+for displaying the status of each database migration.
+
 ## Upgrading to a new major version
 
 Major versions are reserved for backwards incompatible changes. We recommend that