std::fmt needs documentation on how to implement the traits

[chris-morgan]

OK, so we have our fancy new formatting with its traits and all, but at present it's not obvious as (most commonly) a library developer how to implement its goodness for your own types.

We should have a new section in the std::fmt documentation on "how to implement these traits". I think the general advice should be to use write!. It could have an example like this:

use std::fmt;

struct Vector2D {
    x: int,
    y: int,
}

impl fmt::Default for Vector2D {
    fn fmt(obj: &Vector2D, f: &mut fmt::Formatter) {
        write!(f.buf, "({}, {})", obj.x, obj.y)
    }
}

(I still hate the way it's a static method with an explicit first argument rather than a regular method with &self. But ah! there is no syntax to express the other at present.)

This is a very basic example; more will be needed for the adventurous, covering flags, padding and so forth, how f.buf is a Writer which means you're dealing a lot with bytes rather than strings, etc.

This is related to #9806 in that it will make the way toward removing ToStr plainer.