chore: update cluster_hocon_file doc

Co-authored-by: Zaiming (Stone) Shi <zmstone@gmail.com>

apps/emqx/src/emqx_config.erl

@@ -530,6 +530,7 @@ deprecated_conf_file(Opts) when is_map(Opts) ->
 deprecated_conf_file(Which) when is_atom(Which) ->
     application:get_env(emqx, Which, undefined).
 
+%% The newer version cluster-wide config persistence file.
 cluster_hocon_file(Opts) when is_map(Opts) ->
     Key =
         case maps:get(override_to, Opts, cluster) of