doc/go1.20: add section on coverage
    
    Add some basic material on the changes to code coverage testing
    to the release notes.
    
    For #54202.
    
    Change-Id: I28200d43b4952ce8e8ecf46c8fe8e97c81d245e5
    Reviewed-on: https://go-review.googlesource.com/c/go/+/453857
    TryBot-Result: Gopher Robot <gobot@golang.org>
    Run-TryBot: Than McIntosh <thanm@google.com>
    Reviewed-by: Eli Bendersky <eliben@google.com>
    Reviewed-by: Cherry Mui <cherryyz@google.com>