specify what happens with "service" in the PAM client

README.md

@@ -189,6 +189,8 @@ The module knows about the following options:
 * `shard=`*ID* sets the *shard* parameter, which may be used by the
   authentication server to limit authentication to a subset of users.
 
+The `service` parameter sent to the auth server will be the PAM service,
+it is not possible to override this in the PAM configuration.
 
 ## NGINX mail_http_auth support