Compound assignment desugaring doesn't take into account autoderef

In the reference, compound assignment expressions with non-primitive operands are described as being desugared into method calls. That is, x += y is turned into x.add_assign(y).

This desugaring is incorrect. The += form does not do autoderef, while the .add_assign form does autoderef. For example:

use std::ops::AddAssign;
fn foo<T: AddAssign<U>, U>(x: &mut &mut T, y: U) {
    // This compiles
    x.add_assign(y);
    // This doesn't compile
    x += y;
}

Start with the compound-assignment section linked in the Rust Reference and compare its stated desugaring with the provided autoderef example. Verify the behavior against the example, then update the reference text so it accurately distinguishes += from an .add_assign method call; done when the documentation no longer implies equivalent autoderef behavior.