docs: drop healthcheck comment; rephrase release note POSTGRES_USER warning + check command

compose.yml

@@ -107,9 +107,6 @@ services:
       interval: 30s
       timeout: 10s
       retries: 5
-      # generous: on a major-version bump the image installs the old binaries and
-      # runs pg_upgrade on first boot before the server accepts connections —
-      # don't let the healthcheck kill an in-progress migration
       start_period: 15m
     deploy:
       labels:

release/1.0.0+3.5.3

@@ -9,12 +9,11 @@ Rename these in your app's .env (the values carry over):
   DISCOURSE_SMTP_AUTH     --> DISCOURSE_SMTP_AUTHENTICATION
   DISCOURSE_SMTP_PROTOCOL --> DISCOURSE_SMTP_ENABLE_START_TLS (takes a boolean true/false, not the old tls/ssl value, so translate it rather than copying it straight across)
 
-The database now runs the discourse/postgres image, which upgrades your existing
-postgres cluster to a new major version in place on first boot (no dump/restore).
+WARNING: if your deployment's database has an "install user" other than `postgres`
+(some older deployments do), you must set the POSTGRES_USER env var in your .env
+for this migration, otherwise the postgres upgrade aborts with an install-user
+mismatch.
 
-That upgrade runs as the cluster's bootstrap superuser ("install user"), which the
-recipe sets via POSTGRES_USER, defaulting to `postgres`. This default is correct
-for clusters created by this recipe and the old bitnami recipe, so most operators
-need to do nothing. ONLY if your cluster was bootstrapped with a different
-superuser (e.g. `discourse`) must you set POSTGRES_USER in your app's .env before
-upgrading -- otherwise pg_upgrade aborts with an install-user mismatch.
+Check your old deployment's install user before upgrading:
+
+  abra app run YOURAPPDOMAIN db -- psql -U discourse -tAc 'select rolname from pg_roles where oid = 10'