Clarify range's exclusivity.

Inspired by http://www.reddit.com/r/rust/comments/298js2/what_is_the_rationale_behind_the_second_parameter/
We shouldn't display antipatterns as examples -- it's confusing for users.

It's just a convention and I'm not sure we need to mention array indexing -- maybe instead we can mention the iteration length is `stop - start`?