[radio] update `otPlatRadioSetRxOnWhenIdle` documentation #475

Job	Run time
backbone-router	13m 45s
BR (mDNSResponder, TREL=0)	30m 17s
BR (mDNSResponder, TREL=1)	30m 20s
BR MATN (mDNSResponder, TREL=0)	19m 16s
BR LowPower (mDNSResponder, TREL=0)	8m 30s
BR nat64 (mDNSResponder, TREL=0)	23m 58s
BR (avahi, TREL=0)	30m 33s
BR (avahi, TREL=1)	29m 57s
BR core-firewall (avahi, TREL=0)	29m 53s
upload-coverage	47s
	3h 37m 16s

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