Add documentation for `multi_cartesian_product` when iterator is empty #662

Job	Run time
check format	10s
msrv	45s
test	55s
check	11s
check (--no-default-features)	14s
check (--no-default-features --features use_alloc)	12s
check (--all-targets --all-features)	23s
All checks succeeded	0s
	2m 50s

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