clarifying iterator trait documentation
auto merge of #15897 : Gankro/rust/it-docs, r=kballard

I found these things to be ambiguous, or at least worth stating explicitly to reduce the amount a user/developer needs to think about the API.
Auto-merging c4209d1

Travis: Passed at 2014-07-23T07:44:27Z
https://travis-ci.org/vhbit/rust/builds/30624564