service: reformat postgres_tag description in argspec

roles/service/meta/argument_specs.yaml

@@ -139,9 +139,10 @@ argument_specs:
           - none
         default: none
       service_postgres_tag:
-        description: >
-          Postgresql version to use. Can be debian (n) or alpine-based (n-alpine), where n can be major version like 14 or minor like 14.13.
-          Required if service_database_type is postgres.
+        description:
+          - Postgresql version to use.
+          - Can be debian (n) or alpine-based (n-alpine), where n can be major version like 14 or minor like 14.13.
+          - Required if service_database_type is postgres, does nothing otherwise
         type: str
         required: false