doc/go1.20: relnote and take care of TODOs
    
    The main change here is documenting the last-minute addition types.Satisfies.
    
    Change-Id: I8be2d2ad730ba108706bd849b68cbd1f4d68a4b8
    Reviewed-on: https://go-review.googlesource.com/c/go/+/455698
    Run-TryBot: Russ Cox <rsc@golang.org>
    TryBot-Result: Gopher Robot <gobot@golang.org>
    Reviewed-by: Heschi Kreinick <heschi@google.com>