Add gitaly cluster geo test to validation doc

doc/administration/geo/replication/geo_validation_tests.md

@@ -145,3 +145,15 @@ The following are PostgreSQL upgrade validation tests we performed.
   - [`gitlab-ctl` reconfigure fails on Redis node in multi-node Geo setup](https://gitlab.com/gitlab-org/omnibus-gitlab/-/issues/4706).
   - [Geo multi-node upgrade from 12.0.9 to 12.1.9 does not upgrade PostgreSQL](https://gitlab.com/gitlab-org/omnibus-gitlab/-/issues/4705).
   - [Refresh foreign tables fails on app server in multi-node setup after upgrade to 12.1.9](https://gitlab.com/gitlab-org/gitlab/-/issues/32119).
+
+## Other tests
+
+The following are additional validation tests we performed.
+
+### August 2020
+
+[Test Gitaly Cluster on a Geo Deployment](https://gitlab.com/gitlab-org/gitlab/-/issues/223210):
+
+- Description: Tested a Geo deployment with Gitaly clusters configured on both the primary and secondary Geo sites. Triggered automatic Gitaly cluster failover on the primary Geo site, and ran end-to-end Geo tests. Then triggered Gitaly cluster failover on the secondary Geo site, and re-ran the end-to-end Geo tests.
+  
+- Outcome: Successful end-to-end tests before and after Gitaly cluster failover on the primary site, and before and after Gitaly cluster failover on the secondary site.