multiboot2-common: improve documentation #1053

Job	Run time
build (nightly) / rust	1m 7s
build (msrv) / rust	21s
build (stable) / rust	15s
style (stable) / rust	19s
build no_std (stable) [w/o builder] / rust	24s
build no_std (stable) / rust	14s
build no_std (stable) [Windows] / rust	2m 12s
build no_std (msrv) / rust	19s
style (msrv) / rust	21s
build no_std (nightly) / rust	47s
tests with miri (nightly) / rust	1m 55s
style (nightly) / rust	22s
	8m 36s

Provide feedback