Update docs/maintenance-postgres.md: add a note about upgrading PostgreSQL

PostgreSQL major version upgrade can cause a service on the homeserver to be disrupted in some cases. The notice should encourage casual inexperienced homeserver administrators who are going to upgrade PostgreSQL without taking a proper measure against a possible failure to take extra precautions before venturing to upgrade PostgreSQL.

Signed-off-by: Suguru Hirahara <acioustick@noreply.codeberg.org>

docs/maintenance-postgres.md

@@ -73,6 +73,8 @@ Restoring a backup made this way can be done by [importing it](importing-postgre
 
 ## Upgrading PostgreSQL
 
+**Note**: before upgrading PostgreSQL, please take extra precautions such as checking service's PostgreSQL version requirement. There is a possibility that some services do not support the latest version yet. If the service on your homeserver is incompatible with the upgraded version, you will have to wait until the incompatibility is addressed, address it by yourself, or revert the PostgreSQL version to keep using the service.
+
 Unless you are using an [external Postgres server](configuring-playbook-external-postgres.md), this playbook initially installs Postgres for you.
 
 Once installed, the playbook attempts to preserve the Postgres version it starts with. This is because newer Postgres versions cannot start with data generated by older Postgres versions.