multiboot2-common: improve doc #1074

Job	Run time
build (stable) / rust	13s
build (nightly) / rust	25s
build (msrv) / rust	18s
style (stable) / rust	22s
build no_std (stable) / rust	19s
build no_std (stable) [Windows] / rust	1m 37s
build no_std (stable) [w/o builder] / rust	17s
style (msrv) / rust	19s
build no_std (msrv) / rust	25s
build no_std (nightly) / rust	27s
style (nightly) / rust	29s
tests with miri (nightly) / rust	1m 46s
	6m 57s

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