Mention the new behaviour on unsafe database locale in the upgrade notes

changelog.d/12288.misc

@@ -0,0 +1 @@
+Refuse to start if DB has non-`C` locale, unless config flag `allow_unsafe_db_locale` is set to true.

docs/upgrade.md

@@ -99,6 +99,13 @@ experimental_features:
   groups_enabled: false
 ```
 
+## Change in behaviour for PostgreSQL databases with unsafe locale
+
+Synapse now refuses to start when using PostgreSQL with non-`C` values for `COLLATE` and
+`CTYPE` unless the config flag `allow_unsafe_locale`, found in the database section of
+the configuration file, is set to `true`. See the [PostgreSQL documentation](https://matrix-org.github.io/synapse/latest/postgres.html#fixing-incorrect-collate-or-ctype)
+for more information and instructions on how to fix a database with incorrect values.
+
 # Upgrading to v1.55.0
 
 ## `synctl` script has been moved