AC_AttitudeControl: add comments for system ID disturbance input code #868

Job	Run time
build (stm32f7, chibios, 10)	14m 8s
build (stm32h7, chibios, 10)	12m 41s
build (fmuv2-plane, chibios, 10)	4m 10s
build (periph-build, chibios, 10)	18m 49s
build (iofirmware, chibios, 10)	2m 34s
build (CubeOrange-bootloader, chibios, 10)	1m 10s
build (fmuv3-bootloader, chibios, 10)	1m 27s
build (revo-bootloader, chibios, 10)	1m 7s
build (stm32h7-debug, chibios, 10)	7m 14s
build (fmuv3, chibios, 10)	13m 49s
build (revo-mini, chibios, 10)	10m 34s
build (MatekF405-Wing, chibios, 10)	11m 10s
build (CubeOrange-ODID, chibios, 10)	8m 22s
build (CubeRedPrimary-bootloader, chibios, 10)	1m 14s
build (configure-all, chibios, 10)	9m 16s
build (build-options-defaults-test, chibios, 10)	19m 38s
build (signing, chibios, 10)	4m 10s
build (CubeOrange-PPP, chibios, 10)	6m 47s
build (CubeOrange-SOHW, chibios, 10)	6m 59s
build (Pixhawk6X-PPPGW, chibios, 10)	3m 24s
	2h 38m 43s

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