`iterate`: document a possible panic #686

Job	Run time
doc	7s
check	8s
msrv	37s
check (--no-default-features)	20s
check format	4s
check (--no-default-features --features use_alloc)	6s
test	48s
check (--all-targets --all-features)	20s
All checks succeeded	1s
	2m 31s

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