Add comments for battery_* functions on OpenBSD

config.def.h

@@ -13,7 +13,9 @@ static const char unknown_str[] = "n/a";
  * function             description                     argument (example)
  *
  * battery_perc         battery percentage              battery name (BAT0)
+ *                                                      NULL on OpenBSD
  * battery_state        battery charging state          battery name (BAT0)
+ *                                                      NULL on OpenBSD
  * cpu_perc             cpu usage in percent            NULL
  * cpu_freq             cpu frequency in MHz            NULL
  * datetime             date and time                   format string (%F %T)