`iterate`: document a possible panic #687

Job	Run time
doc	6s
msrv	48s
check format	4s
check	20s
check (--no-default-features)	6s
test	55s
check (--no-default-features --features use_alloc)	8s
check (--all-targets --all-features)	21s
All checks succeeded	1s
	2m 49s

Provide feedback