Adjust importing-postgres docs to mention postgres_default_import_database

docs/importing-postgres.md

@@ -24,11 +24,14 @@ To import, run this command (make sure to replace `<server-path-to-postgres-dump
 
 ```sh
 ansible-playbook -i inventory/hosts setup.yml \
---extra-vars='server_path_postgres_dump=<server-path-to-postgres-dump.sql>' \
+--extra-vars='server_path_postgres_dump=<server-path-to-postgres-dump.sql> postgres_default_import_database=matrix' \
 --tags=import-postgres
 ```
 
-**Note**: `<server-path-to-postgres-dump.sql>` must be a file path to a Postgres dump file on the server (not on your local machine!).
+**Notes**:
+
+- `<server-path-to-postgres-dump.sql>` must be a file path to a Postgres dump file on the server (not on your local machine!)
+- `postgres_default_import_database` defaults to `matrix`, which is useful for importing multiple databases (for dumps made with `pg_dumpall`). If you're importing a single database (e.g. `synapse`), consider changing `postgres_default_import_database` accordingly
 
 
 ## Troubleshooting