Plane: Doc. update for ALT_HOLD_RTL with Rally Points.

ArduPlane/Parameters.pde

@@ -675,7 +675,7 @@ const AP_Param::Info var_info[] PROGMEM = {
 
     // @Param: ALT_HOLD_RTL
     // @DisplayName: RTL altitude
-    // @Description: Return to launch target altitude. This is the altitude the plane will aim for and loiter at when returning home. If this is negative (usually -1) then the plane will use the current altitude at the time of entering RTL.
+    // @Description: Return to launch target altitude. This is the altitude the plane will aim for and loiter at when returning home. If this is negative (usually -1) then the plane will use the current altitude at the time of entering RTL. Note that when transiting to a Rally Point the alitude of the Rally Point is used instead of ALT_HOLD_RTL.
     // @Units: centimeters
     // @User: User
     GSCALAR(RTL_altitude_cm,        "ALT_HOLD_RTL",   ALT_HOLD_HOME_CM),