Document usbhid-ups "lowbatt" option

[clepple]

As of about commit 746cb86110982920b9db8f5066b766d560d19eef, the lowbatt driver option for usbhid-ups is not documented in the man page (though it shows up in usbhid-ups -h, and in other man pages such as mge-shut(8)).

We should probably cross-check the rest of the list of options.

-x lowbatt=<value>
-x offdelay=<value>
-x ondelay=<value>
-x pollfreq=<value>
-x pollonly
-x onlinedischarge
-x onlinedischarge_onbattery
-x onlinedischarge_calibration
-x onlinedischarge_log_throttle_sec=<value>
-x onlinedischarge_log_throttle_hovercharge=<value>
-x disable_fix_report_desc
-x subdriver=<value>
-x vendor=<value>
-x product=<value>
-x serial=<value>
-x vendorid=<value>
-x productid=<value>
-x bus=<value>
-x device=<value>
-x busport=<value>
-x allow_duplicates
-x usb_set_altinterface=<value>
-x explore
-x maxreport
-x interruptonly
-x interruptsize=<value>
-x waitbeforereconnect=<value>