Add comment to mautrix-facebook bridge regarding alembic migrations

roles/matrix-bridge-mautrix-facebook/templates/systemd/matrix-mautrix-facebook.service.j2

@@ -15,6 +15,11 @@ Type=simple
 Environment="HOME={{ matrix_systemd_unit_home_path }}"
 ExecStartPre=-{{ matrix_host_command_docker }} kill matrix-mautrix-facebook
 ExecStartPre=-{{ matrix_host_command_docker }} rm matrix-mautrix-facebook
+
+# This bridge uses another mechanism for migrations now (migrations happen automatically during regular startup),
+# so going forward, running this alembic stuff will not necessary.
+# People who are upgrading from an older version of the bridge should go through this migration
+# first though, so we're keeping it around for now.
 ExecStartPre={{ matrix_host_command_docker }} run --rm --name matrix-mautrix-facebook-db \
 			--log-driver=none \
 			--user={{ matrix_user_uid }}:{{ matrix_user_gid }} \