[radio] update `otPlatRadioSetRxOnWhenIdle` documentation #531

Job	Run time
thread-1-3-gcc-m32	10m 6s
thread-1-3-gcc-m64	10m 28s
thread-1-3-clang-10-m32	10m 8s
thread-1-3-clang-10-m64	9m 28s
channel-manager-csl	12m 4s
expects	30m 38s
packet-verification-1-1-on-1-3	10m 45s
packet-verification-low-power	9m 5s
thread-1-3-posix	9m 16s
upload-coverage	39s
	1h 52m 37s

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