VFR_HUD: edit airspeed comment to also allow CAS if available instead…

… of IAS (mavlink#1375)

message_definitions/v1.0/common.xml

@@ -4133,7 +4133,7 @@
     </message>
     <message id="74" name="VFR_HUD">
       <description>Metrics typically displayed on a HUD for fixed wing aircraft.</description>
-      <field type="float" name="airspeed" units="m/s">Current indicated airspeed (IAS).</field>
+      <field type="float" name="airspeed" units="m/s">Vehicle speed in form appropriate for vehicle type. For standard aircraft this is typically calibrated airspeed (CAS) or indicated airspeed (IAS) - either of which can be used by a pilot to estimate stall speed.</field>
       <field type="float" name="groundspeed" units="m/s">Current ground speed.</field>
       <field type="int16_t" name="heading" units="deg">Current heading in compass units (0-360, 0=north).</field>
       <field type="uint16_t" name="throttle" units="%">Current throttle setting (0 to 100).</field>