multiboot2: various small fixes and doc improvements #1088

Job	Run time
build (msrv) / rust	22s
build (nightly) / rust	1m 2s
build (stable) / rust	45s
build no_std (msrv) / rust	19s
style (msrv) / rust	21s
build no_std (stable) [w/o builder] / rust	20s
build no_std (stable) / rust	38s
style (stable) / rust	41s
build no_std (stable) [Windows] / rust	3m 10s
build no_std (nightly) / rust	36s
tests with miri (nightly) / rust	2m 2s
style (nightly) / rust	51s
	11m 7s

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