doc: Confusing information regarding mutability for str primitive.

[Ryman]

In http://doc.rust-lang.org/std/str/primitive.str.html under the heading of Mutability it seems to say that there are ways to mutably edit a string.To quote:

Many languages have immutable strings by default, and Rust has a particular flavor on this idea. As with the rest of Rust types, strings are immutable by default. If a string is declared as mut, however, it may be mutated. This works the same way as the rest of Rust's type system in the sense that if there's a mutable reference to a string, there may only be one mutable reference to that string. With these guarantees, strings can easily transition between being mutable/immutable with the same benefits of having mutable strings in other languages.

I think this is confusing to users, is this meant to imply String, or is it discussing mutable slices (which aren't allowed anymore?). I feel like this paragraph is in need of review, or possibly removed/moved to String.

[huonw]

I think it is just talking about the old ~str, so yes, it needs to be removed/moved to String.