docs: name the per-database role and describe the owner-owned queue schema

.env.example

@@ -39,8 +39,9 @@
 # Left commented here so that append does not create a duplicate key. Uncomment
 # and set it directly instead if you prefer to edit the file by hand.
 
-# The container runs the app as `healthlog_app`, a database role it creates on
-# every start that reads and writes rows but cannot change the schema. The
+# The container runs the app as `healthlog_app_<database>_<hash>`, a database
+# role it creates on every start that reads and writes rows but cannot change
+# the schema. The
 # migrations keep the DATABASE_URL user. "off" keeps the app on that user, if
 # you manage database roles yourself (docs/self-hosting/database-role.md).
 # DB_APP_ROLE="off"