multiboot2-common: further improve docs #1054

Job	Run time
build (msrv) / rust	18s
build (nightly) / rust	1m 4s
build (stable) / rust	53s
build no_std (msrv) / rust	21s
style (msrv) / rust	36s
style (stable) / rust	14s
build no_std (stable) [Windows] / rust	2m 25s
build no_std (stable) / rust	24s
build no_std (stable) [w/o builder] / rust	13s
build no_std (nightly) / rust	44s
tests with miri (nightly) / rust	1m 42s
style (nightly) / rust	1m 5s
	9m 59s

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