Mention btree_gist extension requirement

doc/administration/reference_architectures/3k_users.md

@@ -804,10 +804,11 @@ SSH into the **primary node**:
    gitlab-psql -d gitlabhq_production
    ```
 
-1. Enable the `pg_trgm` extension:
+1. Enable the `pg_trgm` and `btree_gist` extensions:
 
    ```shell
    CREATE EXTENSION pg_trgm;
+   CREATE EXTENSION btree_gist;
    ```
 
 1. Exit the database prompt by typing `\q` and Enter.

doc/install/installation.md

@@ -333,7 +333,7 @@ Starting from GitLab 12.1, only PostgreSQL is supported. Since GitLab 13.0, we r
    sudo -u postgres psql -d template1 -c "CREATE USER git CREATEDB;"
    ```
 
-1. Create the `pg_trgm` extension (required for GitLab 8.6+):
+1. Create the `pg_trgm` extension:
 
    ```shell
    sudo -u postgres psql -d template1 -c "CREATE EXTENSION IF NOT EXISTS pg_trgm;"