Clarify range's exclusivity.

Inspired by http://www.reddit.com/r/rust/comments/298js2/what_is_the_rationale_behind_the_second_parameter/
It may be useful to retain the range notation, as it is the clearest way of conveying it for those who do understand it. "ending at `stop`" could lead one to think that the range is inclusive if they do not read the entire thing.