Add comment about ping requests sent even to non-supporting clients

src/mod_ping.erl

@@ -103,6 +103,8 @@ init([Host, Opts]) ->
                                   ?MODULE, iq_ping, IQDisc),
     case SendPings of
         true ->
+	    %% Ping requests are sent to all entities, whether they
+	    %% announce 'urn:xmpp:ping' in their caps or not
             ejabberd_hooks:add(sm_register_connection_hook, Host,
                                ?MODULE, user_online, 100),
             ejabberd_hooks:add(sm_remove_connection_hook, Host,