UPGRADING: Note when upgrade-postgresql should be run w/r/t volumes.

UPGRADING.md

@@ -145,6 +145,10 @@ running the included `./upgrade-postgresql` tool. This will create a
 Docker-managed volume named `postgresql-14` to store its data, and will adjust
 the `docker-compose.yml` file to use that.
 
+You can perform this step either before or after updating to use Docker volumes
+(above). In either case, the updated `docker-compose.yml` will use a new Docker
+volume for the upgraded PostgreSQL 14 data.
+
 If the tool does not work, or you would prefer to perform the steps manually,
 see the steps below. These instructions assume that you have not changed the
 default Postgres data path (`/opt/docker/zulip/postgresql/data`) in your