document `where` keyword

Inspired by a short dialog on irc:

20:04 kraemahz How do I tell rust the type of an iterator? <T: Iterator<usize>>(iter: T) and <T: Iterator>(iter: T<usize>) don't work
20:05 Mutabah kraemahz: Iterator<Item=usize>
20:05 kraemahz thanks
20:06 Yurume which is short for fn foo<T: Iterator>(iter: T) where <T as Iterator>::Item == usize afaik.

The where keyword is not mentioned in Iterators, Generics, or Traits docs despite its use in base types like Iterator.

[steveklabnik]

Yup, this is on my list. Thanks for the issue.

[jroesch]

cc me