MAV_CMD_NAV_LOITER_TIME: add Heading Required field (mavlink#1383)

message_definitions/v1.0/common.xml

@@ -1075,11 +1075,11 @@
         <param index="7" label="Altitude" units="m">Altitude</param>
       </entry>
       <entry value="19" name="MAV_CMD_NAV_LOITER_TIME" hasLocation="true" isDestination="true">
-        <description>Loiter around this waypoint for X seconds</description>
-        <param index="1" label="Time" units="s" minValue="0">Loiter time.</param>
-        <param index="2">Empty</param>
-        <param index="3" label="Radius" units="m">Radius around waypoint. If positive loiter clockwise, else counter-clockwise.</param>
-        <param index="4" label="Xtrack Location">Forward moving aircraft this sets exit xtrack location: 0 for center of loiter wp, 1 for exit location. Else, this is desired yaw angle.  NaN to use the current system yaw heading mode (e.g. yaw towards next waypoint, yaw to home, etc.).</param>
+        <description>Loiter at the specified latitude, longitude and altitude for a certain amount of time. Multicopter vehicles stop at the point (within a vehicle-specific acceptance radius). Forward-moving vehicles (e.g. fixed-wing) circle the point with the specified radius/direction. If the Heading Required parameter (2) is non-zero forward moving aircraft will only leave the loiter circle once heading towards the next waypoint.</description>
+        <param index="1" label="Time" units="s" minValue="0">Loiter time (only starts once Lat, Lon and Alt is reached).</param>
+        <param index="2" label="Heading Required" minValue="0" maxValue="1" increment="1">Leave loiter circle only once heading towards the next waypoint (0 = False)</param>
+        <param index="3" label="Radius" units="m">Loiter radius around waypoint for forward-moving vehicles. If positive loiter clockwise, else counter-clockwise.</param>
+        <param index="4" label="Xtrack Location">For forward-moving aircraft this sets exit xtrack location: 0 for center of loiter wp, 1 for exit location. Else, this is desired yaw angle. NaN to use the current system yaw heading mode (e.g. yaw towards next waypoint, yaw to home, etc.).</param>
         <param index="5" label="Latitude">Latitude</param>
         <param index="6" label="Longitude">Longitude</param>
         <param index="7" label="Altitude" units="m">Altitude</param>