Improve Description of EKF2_GND_EFF_MAX_HGT Parameter

Working on automatic landings I have found that the parameter EKF2_GND_EFF_MAX_HGT is only used if there is a range sensor or other terrain data estimation source.

I would recommend that the parameter description be updated to correctly reflect this and match the documentation at ECL/EKF Overview & Tuning · PX4 v1.9.0 User Guide.

I’m curious as to why not assume home altitude for ground effect estimation as we do with MPC_LAND_ALT1 and MPC_LAND_ALT2 (which uses dist_to_ground) when no terrain data is present? Currently land_detector just checks if the aircraft is descending straight down and assumes there is a ground effect (which can cause big altitude errors to accumulate).

If we only enabled Ground Effect at EKF2_GND_EFF_MAX_HGT above HOME altitude there is an edge case where landing could occur on a hill at higher elevation and ground effect would be ignored - but MPC_LAND_ALT1/2 would already be ignored and the aircraft would descend at MPC_Z_VEL_MAX_DN. For the majority of use cases (RTL and Mission Completion) autonomous landing would be improved.