[radio] update `otPlatRadioSetRxOnWhenIdle` documentation #473

Job	Run time
backbone-router	13m 35s
BR (mDNSResponder, TREL=0)	30m 15s
BR (mDNSResponder, TREL=1)	30m 31s
BR MATN (mDNSResponder, TREL=0)	19m 31s
BR LowPower (mDNSResponder, TREL=0)	8m 33s
BR nat64 (mDNSResponder, TREL=0)	24m 33s
BR (avahi, TREL=0)	30m 9s
BR (avahi, TREL=1)	30m 34s
BR core-firewall (avahi, TREL=0)	29m 46s
upload-coverage	0s
	3h 37m 27s

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