Add documentation for `multi_cartesian_product` when iterator is empty #752

Job	Run time
msrv	44s
check format	6s
check	12s
doc	11s
check (--no-default-features)	7s
semver-checks	22s
check (--no-default-features --features use_alloc)	11s
test	57s
check (--all-targets --all-features)	17s
All checks succeeded	1s
	3m 8s

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