ekf2: Update parameter description

src/modules/ekf2/ekf2_params.c

@@ -1079,7 +1079,12 @@ PARAM_DEFINE_FLOAT(EKF2_MAGB_K, 0.2f);
  *
  * If this parameter is enabled then the estimator will make use of the range finder measurements
  * to estimate it's height even if range sensor is not the primary height source. It will only do so if conditions
- * for range measurement fusion are met.
+ * for range measurement fusion are met. This enables the range finder to be used during low speed and low altitude
+ * operation. Speed and height criteria are controlled by EKF2_RNG_A_VMAX and EKF2_RNG_A_HMAX.
+ * It should not be used for terrain following. It is intended to be used where a vertical takeoff and landing
+ * is performed, and horizontal flight does not occur until above EKF2_RNG_A_HMAX. If vehicle motion causes
+ * repeated switvhing between the rimary height sensor and range finder, an offset in the local position origin
+ * can accumulate. For terrain following, it is recommended to use the MPC_ALT_MODE parameter instead.
  *
  * @group EKF2
  * @value 0 Range aid disabled