AP_NavEKF3: Clarify intended use of setTerrainHgtStable accessor

libraries/AP_NavEKF3/AP_NavEKF3.cpp

@@ -1463,7 +1463,8 @@ void NavEKF3::setTouchdownExpected(bool val)
 
 // Set to true if the terrain underneath is stable enough to be used as a height reference
 // in combination with a range finder. Set to false if the terrain underneath the vehicle
-// cannot be used as a height reference
+// cannot be used as a height reference. Use to prevent range finder operation otherwise
+// enabled by the combination of EK3_RNG_AID_HGT and EK3_RNG_USE_SPD parameters.
 void NavEKF3::setTerrainHgtStable(bool val)
 {
     if (core) {

libraries/AP_NavEKF3/AP_NavEKF3.h

@@ -316,7 +316,8 @@ public:
 
     // Set to true if the terrain underneath is stable enough to be used as a height reference
     // in combination with a range finder. Set to false if the terrain underneath the vehicle
-    // cannot be used as a height reference
+    // cannot be used as a height reference. Use to prevent range finder operation otherwise
+    // enabled by the combination of EK3_RNG_AID_HGT and EK3_RNG_USE_SPD parameters.
     void setTerrainHgtStable(bool val);
 
     /*

libraries/AP_NavEKF3/AP_NavEKF3_VehicleStatus.cpp

@@ -466,7 +466,8 @@ void NavEKF3_core::setTouchdownExpected(bool val)
 
 // Set to true if the terrain underneath is stable enough to be used as a height reference
 // in combination with a range finder. Set to false if the terrain underneath the vehicle
-// cannot be used as a height reference
+// cannot be used as a height reference. Use to prevent range finder operation otherwise
+// enabled by the combination of EK3_RNG_AID_HGT and EK3_RNG_USE_SPD parameters.
 void NavEKF3_core::setTerrainHgtStable(bool val)
 {
     terrainHgtStableSet_ms = imuSampleTime_ms;

libraries/AP_NavEKF3/AP_NavEKF3_core.h

@@ -325,7 +325,8 @@ public:
 
     // Set to true if the terrain underneath is stable enough to be used as a height reference
     // in combination with a range finder. Set to false if the terrain underneath the vehicle
-    // cannot be used as a height reference
+    // cannot be used as a height reference. Use to prevent range finder operation otherwise
+    // enabled by the combination of EK3_RNG_AID_HGT and EK3_RNG_USE_SPD parameters.
     void setTerrainHgtStable(bool val);
 
     /*