Compass: Updated COMPASS_PRIMARY parameter description

libraries/AP_Compass/Compass.cpp

@@ -104,7 +104,7 @@ const AP_Param::GroupInfo Compass::var_info[] PROGMEM = {
 
     // @Param: PRIMARY
     // @DisplayName: Choose primary compass
-    // @Description: If more than one compass is available this selects which compass is the primary.
+    // @Description: If more than one compass is available this selects which compass is the primary. Normally 0=External, 1=Internal. If no External compass is attached this parameter is ignored
     // @Values: 0:FirstCompass,1:SecondCompass
     // @User: Advanced
     AP_GROUPINFO("PRIMARY", 12, Compass, _primary, 0),