multiboot2-common: improve documentation #1055

Job	Run time
build (nightly) / rust	38s
build (stable) / rust	54s
build (msrv) / rust	1m 6s
tests with miri (nightly) / rust	1m 37s
style (nightly) / rust	19s
build no_std (nightly) / rust	19s
build no_std (stable) / rust	13s
style (stable) / rust	14s
build no_std (stable) [Windows] / rust	3m 22s
build no_std (stable) [w/o builder] / rust	14s
build no_std (msrv) / rust	34s
style (msrv) / rust	53s
	10m 23s

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