Geo: Document promotion-preflight-checks command

doc/administration/geo/disaster_recovery/index.md

@@ -122,7 +122,11 @@ Note the following when promoting a secondary:
    roles ['geo_secondary_role']
    ```
 
-1. Promote the **secondary** node to the **primary** node. Execute:
+1. Promote the **secondary** node to the **primary** node.
+
+   Before promoting a secondary node to primary, preflight checks should be run. They can be run separately or along with the promotion script.
+
+   To promote the secondary node to primary along with preflight checks:
 
    ```shell
    gitlab-ctl promote-to-primary-node
@@ -134,6 +138,12 @@ Note the following when promoting a secondary:
    gitlab-ctl promote-to-primary-node --skip-preflight-check
    ```
 
+   You can also run preflight checks separately:
+
+   ```shell
+   gitlab-ctl promotion-preflight-checks
+   ```
+
 1. Verify you can connect to the newly promoted **primary** node using the URL used
    previously for the **secondary** node.
 1. If successful, the **secondary** node has now been promoted to the **primary** node.

doc/administration/geo/disaster_recovery/planned_failover.md

@@ -45,8 +45,13 @@ be found in `/var/opt/gitlab/gitlab-rails/shared/pages` if using Omnibus).
 
 ## Preflight checks
 
-Follow these steps before scheduling a planned failover to ensure the process
-will go smoothly.
+Run this command to list out all preflight checks before scheduling a planned failover to ensure the process will go smoothly:
+
+```shell
+gitlab-ctl promotion-preflight-checks
+```
+
+Each step is described in more detail below.
 
 ### Object storage