fixup #17188: Add most likely reasons for clock going backwards

Add "You might have an empty clock battery or bad NTP server."

src/or/statefile.c

@@ -381,7 +381,8 @@ or_state_load(void)
       format_iso_time(now_str, time(NULL));
       log_warn(LD_GENERAL, "Your system clock has been set back in time. "
                "Tor needs an accurate clock to know when the consensus "
-               "expires. Clock time is %s, state file time is %s.",
+               "expires. You might have an empty clock battery or bad NTP "
+               "server. Clock time is %s, state file time is %s.",
                now_str, last_written_str);
     }
   } else {