doc/go1.20: delete remaining TODO
    
    This section is complete.
    
    For #54202.
    
    Change-Id: I304cc55a5b8ed53e8b8dff73a5feb5ef39207846
    Reviewed-on: https://go-review.googlesource.com/c/go/+/455895
    Run-TryBot: Michael Pratt <mpratt@google.com>
    Reviewed-by: Carlos Amedee <carlos@golang.org>
    TryBot-Result: Gopher Robot <gobot@golang.org>