Add instructions for enabling the `pg_trgm` extension in the production db. As...

Add instructions for enabling the `pg_trgm` extension in the production db. As a workaround to [a fault during HA setup](https://gitlab.com/gitlab-org/omnibus-gitlab/issues/2501).

doc/administration/high_availability/database.md

@@ -97,9 +97,12 @@ If you use a cloud-managed service, or provide your own PostgreSQL:
     Enter new password:
     Enter it again:
     ```
-
-1. Enable the `pg_trgm` extension:
+1. Exit from editing `template1` prompt by typing `\q` and Enter.
+1. Enable the `pg_trgm` extension within the `gitlabhq_production` database:
+    
     ```
+    gitlab-psql -d gitlabhq_production
+    
     CREATE EXTENSION pg_trgm;
 
     # Output: