common.xml: description and unit fixes

[peterbarker]

Patches brought in from mavlink/mavlink/master

When ArduPilot uses this as its reference commit it is a no-compiler-output change:

Board                    AP_Periph  antennatracker  blimp  bootloader  copter  heli  iofirmware  plane  rover  sub
CubeOrange-periph-heavy  *                                 *                                                   
Durandal                            *               *      *           *       *                 *      *      *
Hitec-Airspeed           *                                 *                                                   
KakuteH7-bdshot                     *               *      *           *       *                 *      *      *
MatekF405                           *               *      *           *       *                 *      *      *
Pixhawk1-1M-bdshot                  *               *                  *       *                 *      *      *
f103-QiotekPeriph        *                                 *                                                   
f303-Universal           *                                 *                                                   
iomcu                                                                                *                         
revo-mini                           *               *      *           *       *                 *      *      *
skyviper-v2450                                                         *