ArduSub: annotate and correct parameter conversion comments

Adds the missing PARAMETER_CONVERSION annotation to the conversion_table
(battery failsafe parameters, Mar-2018, and COMPASS_ENABLE, Apr-2019)
and to the attitude control _FILT to FLTE conversion (Jul-2019).
Remaining changes are date format corrections.  No functional change.

Also annotates the ARMDISARM_UNUSED to ARMDISARM rewrite of stored
RCn_OPTION values, which calls set_and_save() directly rather than going
through an AP_Param::convert_* helper and so was missed by an audit
anchored on those helpers.  It went in during Sep-2021, but Sub has no
4.2 to 4.4 releases, so established by content it first shipped in
ArduSub-4.5.0.

ArduSub/system.cpp

@@ -48,6 +48,7 @@ void Sub::init_ardupilot()
     gcs().setup_uarts();
 
     // initialise rc channels including setting mode
+    // PARAMETER_CONVERSION - Added: Sep-2021 for ArduSub-4.5
     rc().convert_options(RC_Channel::AUX_FUNC::ARMDISARM_UNUSED, RC_Channel::AUX_FUNC::ARMDISARM);
     rc().init();