Improve FlashStorage documentation #15849

Job	Run time
docs	14m 11s
msrv	11m 45s
esp-hal	17m 32s
link-check	21s
detect-extras-runner / detect-runner	4s
detect-host-tests-runner / detect-runner	3s
extras	2m 35s
host-tests	3m 19s
	49m 50s

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