Add docs for application_settings_cache_seconds

doc/administration/application_settings_cache.md

+---
+stage: Enablement
+group: Memory
+info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
+---
+
+# Changing application settings cache expiry interval **(CORE ONLY)**
+
+Application settings are cached for 60 seconds by default which should work
+for most installations. A higher value would mean a greater delay between
+changing an application setting and noticing that change come into effect.
+A value of `0` would result in the `application_settings` table being
+loaded for every request causing extra load on Redis and/or PostgreSQL.
+It is therefore recommended to keep the value above zero.
+
+## Change the application settings cache expiry
+
+To change the expiry value:
+
+**For Omnibus installations**
+
+1. Edit `/etc/gitlab/gitlab.rb`:
+
+   ```ruby
+   gitlab_rails['application_settings_cache_seconds'] = 60
+   ```
+
+1. Save the file, and reconfigure and restart GitLab for the changes to take effect:
+
+   ```shell
+   gitlab-ctl reconfigure
+   gitlab-ctl restart
+   ```
+
+---
+
+**For installations from source**
+
+1. Edit `config/gitlab.yml`:
+
+   ```yaml
+   gitlab:
+     application_settings_cache_seconds: 60
+   ```
+
+1. Save the file and [restart](restart_gitlab.md#installations-from-source)
+   GitLab for the changes to take effect.

doc/administration/index.md

@@ -80,6 +80,7 @@ Learn how to install, configure, update, and maintain your GitLab instance.
   emails with S/MIME.
 - [Enabling and disabling features flags](feature_flags.md): how to enable and
   disable GitLab features deployed behind feature flags.
+- [Application settings cache expiry interval](application_settings_cache.md)
 
 #### Customizing GitLab appearance