Merge branch 'docs-update-external-secondary-database' into 'master'

added step to populate fdw tables when using external database

See merge request gitlab-org/gitlab-ee!8113

doc/administration/geo/replication/database.md

@@ -558,6 +558,11 @@ the instructions below:
     ```bash
     gitlab-ctl restart
     ```
+2. Populate the FDW tables
+
+    ```bash
+    gitlab-rake geo:db:refresh_foreign_tables
+    ```
 
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