Give a bit more information about how Plane Guided works

plane/source/docs/guided-mode.rst

@@ -4,12 +4,41 @@
 GUIDED Mode
 ===========
 
-The GUIDED mode is used when you want the aircraft to fly to a specific
-point on the map without setting up a mission. Most ground control
-stations support a "click to fly to" feature where you can click a point
-on the map and the aircraft will fly to that location then loiter.
+GUIDED mode is used when you want the aircraft to fly to a specific
+point on the map, or in specific direction (heading) without setting up a mission. 
 
 The other major use for GUIDED mode is in :ref:`geo-fencing <geofencing>`.
 When the geo-fence is breached the aircraft will enter GUIDED mode, and
 head to the predefined geo-fence return point, where it will loiter
 until the operator takes over.
+
+Location sub-mode
+-----------------
+Most ground control
+stations support a "click to fly to" feature where you can click a point
+on the map and the aircraft will fly to that location then loiter. If an 
+altitude is requested, the aircraft will try to achieve the target altitude as it flies,
+and will loiter down or up to the requested altitude once it arrives, if required. 
+
+.. image:: ../images/guided-altitude.png
+
+This sub-mode can also be requested via MAVLink using the DO_REPOSITION command.
+
+If the vehicle is flying in Guided "Location sub-mode" then GUIDED_CHANGE_ALTITUDE and GUIDED_CHANGE_SPEED 
+commands will be ignored.
+
+Heading sub-mode
+----------------
+The guided mode heading feature isn't supported by Mission Planner or QGroundControl, but can be
+requested via MAVLink commands or using Lua. If the aircraft receives a GUIDED_CHANGE_HEADING
+command it will fly indefinitely towards the heading in degrees. 
+
+The target altitude and desired airspeed or
+groundspeed of the vehicle to fly at can be set using the GUIDED_CHANGE_ALTITUDE and GUIDED_CHANGE_SPEED
+MAVLink commands. If the aircraft is flying in Guided "Heading sub-mode" then it will try to
+achieve the altitude as it flies, as there is no "end point" to a heading target for a final loiter.
+
+For more information about this, see the MAVLink GUIDED_CHANGE_HEADING documentation 
+https://mavlink.io/en/messages/ardupilotmega.html#MAV_CMD_GUIDED_CHANGE_HEADING
+
+