Guide: ownership

Whew. This section was so important, I saved it for last.

/cc @wycats, @nikomatsakis
You can re-lend a &mut, like this:

    let mut x = 5i;
    let y = &mut x;
    {
        let _reborrow_mut = &mut *y;
    }
    {
        let _reborrow_imm = & *y;
    }