Update UPGRADE notes to include more details about v1.57 upgrade failure mode (#12448)

docs/upgrade.md

@@ -98,6 +98,45 @@ without any risk of reusing transaction IDs.
 Deployments which do not use separate worker processes can be upgraded as normal. Similarly,
 deployments where no applciation services are in use can be upgraded as normal.
 
+<details>
+<summary><b>Recovering from an incorrect upgrade</b></summary>
+
+If the database schema is upgraded *without* stopping the worker responsible
+for AS traffic, then the following error may be given when attempting to start
+a Synapse worker or master process:
+
+```
+**********************************************************************************
+ Error during initialisation:
+
+ Postgres sequence 'application_services_txn_id_seq' is inconsistent with associated
+ table 'application_services_txns'. This can happen if Synapse has been downgraded and
+ then upgraded again, or due to a bad migration.
+
+ To fix this error, shut down Synapse (including any and all workers)
+ and run the following SQL:
+
+     SELECT setval('application_services_txn_id_seq', (
+         SELECT GREATEST(MAX(txn_id), 0) FROM application_services_txns
+     ));
+
+ See docs/postgres.md for more information.
+
+ There may be more information in the logs.
+**********************************************************************************
+```
+
+This error may also be seen if Synapse is *downgraded* to an earlier version,
+and then upgraded again to v1.57.0 or later.
+
+In either case:
+
+ 1. Ensure that the worker responsible for AS traffic is stopped.
+ 2. Run the SQL command given in the error message via `psql`.
+
+Synapse should then start correctly.
+</details>
+
 # Upgrading to v1.56.0
 
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