Rust Book trait section uses '+' without introducing it

[frewsxcv]

http://doc.rust-lang.org/book/traits.html

e.g. fn foo<T: Clone, K: Clone + Debug>(x: T, y: K) {

Might be nice to have a sentence or two describing it

[pjenvey]

The same can be said for rustbyexample where it seems to be introduced in http://rustbyexample.com/bounds.html