Add description about SYS_MC_EST_GROUP in optical flow setup description

[pchatrath]

In http://dev.px4.io/flow_lidar_setup.html or Devguide/flow_lidar_setup.md there is description about using locap position estimator. Hoever untill SYS_MC_EST_GROUP parameter is set to lpe all the lpe parameters mentioned in the documentation won't be visible. So I think setting SYS_MC_EST_GROUP parameter to appropriate setting and doing a reboot should be added to the documentation.

[bkueng]

done!