Clarify important implicit lifetime choice for Iter

[sancho20021]

https://rust-unofficial.github.io/too-many-lists/sixth-combinatorics.html
Here you implement Iter for the linked list assuming it will work, and it works. Good!
But the reader might wonder what is the role of the lifetime 'a? Indeed, we do not see any explicit uses of it except PhantomData.

The use is hidden in the signature of fn iter(&self) -> Iter<T>. We could change it to fn iter<'s>(&self) -> Iter<'s, T> and everything will go wrong as now iterator won't borrow linked list and could be iterated after list's destruction.
I guess this is why we need lifetimes but it is not explained in the section.