hrt_absolute_time: add comment, that time is in us
hrt_absolute_time: add comment, that time is in us

Aerotenna/Firmware,acfloria/Firmware,acfloria/Firmware,jlecoeur/Firmware,krbeverx/Firmware,jlecoeur/Firmware,darknight-007/Firmware,dagar/Firmware,mje-nz/PX4-Firmware,krbeverx/Firmware,jlecoeur/Firmware,mje-nz/PX4-Firmware,dagar/Firmware,darknight-007/Firmware,krbeverx/Firmware,Aerotenna/Firmware,mje-nz/PX4-Firmware,acfloria/Firmware,darknight-007/Firmware,dagar/Firmware,jlecoeur/Firmware,Aerotenna/Firmware,mcgill-robotics/Firmware,PX4/Firmware,mcgill-robotics/Firmware,PX4/Firmware,acfloria/Firmware,jlecoeur/Firmware,dagar/Firmware,Aerotenna/Firmware,mcgill-robotics/Firmware,Aerotenna/Firmware,acfloria/Firmware,mcgill-robotics/Firmware,dagar/Firmware,PX4/Firmware,dagar/Firmware,krbeverx/Firmware,jlecoeur/Firmware,mcgill-robotics/Firmware,mcgill-robotics/Firmware,mje-nz/PX4-Firmware,krbeverx/Firmware,PX4/Firmware,acfloria/Firmware,PX4/Firmware,mje-nz/PX4-Firmware,darknight-007/Firmware,krbeverx/Firmware,mje-nz/PX4-Firmware,PX4/Firmware,krbeverx/Firmware,Aerotenna/Firmware,darknight-007/Firmware,jlecoeur/Firmware,acfloria/Firmware,jlecoeur/Firmware,mje-nz/PX4-Firmware,mcgill-robotics/Firmware,Aerotenna/Firmware,dagar/Firmware,PX4/Firmware