Freezing is outdated

[dalance]

I think freezing by borrow is outdated by NLL.
So scope/borrow/freeze.md may be required to re-written.
In my opinion, freezing can be explained in variable_bindings/freeze.md with the following code.

fn main() {
    let mut _mutable_integer = 7i32;

    {
        // Shadowing by immutable `_mutable_integer`
        let _mutable_integer = _mutable_integer;

        // Error! `_mutable_integer` is frozen in this scope
        _mutable_integer = 50;
        // FIXME ^ Comment out this line

        // `_mutable_integer` goes out of scope
    }

    // Ok! `_mutable_integer` is not frozen in this scope
    _mutable_integer = 3;
}