multiboot2: fill in missing module documentations #764

Job	Run time
build (nightly) / rust	58s
build (stable) / rust	24s
build (msrv) / rust	42s
build no_std (stable) [Windows] / rust	1m 12s
build no_std (stable) / rust	35s
build no_std (stable) [w/o builder] / rust	38s
style (stable) / rust	28s
style (msrv) / rust	29s
build no_std (msrv) / rust	29s
style (nightly) / rust	30s
build no_std (nightly) / rust	34s
tests with miri (nightly) / rust	1m 14s
	8m 13s

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