Blimp: annotate RCn_OPTION parameter conversion

The ARMDISARM_UNUSED to ARMDISARM rewrite of stored RCn_OPTION values had
no annotation: it calls set_and_save() directly rather than going through
an AP_Param::convert_* helper, so an audit anchored on those helpers did
not see it.

Blimp has no release tags, so the release cannot be established by
content the way it is for the other vehicles.  The annotation names the
cycle it was added in instead: Sep-2021, which is 4.2.  No functional
change.

Blimp/system.cpp

@@ -46,6 +46,7 @@ void Blimp::init_ardupilot()
     AP_Param::invalidate_count();
 
     // initialise rc channels including setting mode
+    // PARAMETER_CONVERSION - Added: Sep-2021 for ArduPilot-4.2
     rc().convert_options(RC_Channel::AUX_FUNC::ARMDISARM_UNUSED, RC_Channel::AUX_FUNC::ARMDISARM);
     rc().init();