Add upgrade guidance to database HA docs

Also callout Sidekiq and GitLab restarts after migrations

doc/administration/high_availability/gitlab.md

@@ -131,6 +131,9 @@ need some extra configuration.
 
 1. Run `sudo gitlab-ctl reconfigure` to compile the configuration.
 
+NOTE: **Note:** You will need to restart the GitLab applications nodes after an update has occurred and database
+migrations performed.
+
 ## Enable Monitoring
 
 > [Introduced](https://gitlab.com/gitlab-org/omnibus-gitlab/issues/3786) in GitLab 12.0.

doc/administration/high_availability/sidekiq.md

@@ -94,6 +94,9 @@ you want using steps 1 and 2 from the GitLab downloads page.
 
 1. Run `gitlab-ctl reconfigure`.
 
+NOTE: **Note:** You will need to restart the Sidekiq nodes after an update has occurred and database
+migrations performed.
+
 ## Example configuration
 
 Here's what the ending `/etc/gitlab/gitlab.rb` would look like: