AP_NavEKF2: Clarify intended use of setTerrainHgtStable accessor

libraries/AP_NavEKF2/AP_NavEKF2.cpp

@@ -1315,7 +1315,8 @@ void NavEKF2::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 EK2_RNG_AID_HGT and EK2_RNG_USE_SPD parameters.
 void NavEKF2::setTerrainHgtStable(bool val)
 {
     if (core) {

libraries/AP_NavEKF2/AP_NavEKF2.h

@@ -245,7 +245,8 @@ 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 EK2_RNG_AID_HGT and EK2_RNG_USE_SPD parameters.
     void setTerrainHgtStable(bool val);
 
     /*

libraries/AP_NavEKF2/AP_NavEKF2_VehicleStatus.cpp

@@ -469,7 +469,8 @@ void NavEKF2_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 EK2_RNG_AID_HGT and EK2_RNG_USE_SPD parameters.
 void NavEKF2_core::setTerrainHgtStable(bool val)
 {
     terrainHgtStableSet_ms = imuSampleTime_ms;

libraries/AP_NavEKF2/AP_NavEKF2_core.h

@@ -236,7 +236,8 @@ 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 EK2_RNG_AID_HGT and EK2_RNG_USE_SPD parameters.
     void setTerrainHgtStable(bool val);
 
     /*