doc: 1.20 compiler changes
    
    This adds the nonPGO, non-coverage compiler changes
    for the 1.20 release.  There's not that much user
    visible change.
    
    For #54202.
    
    Change-Id: Ib2964ed5f7e73bb89c720d09b868ab79682f5070
    Reviewed-on: https://go-review.googlesource.com/c/go/+/454536
    TryBot-Result: Gopher Robot <gobot@golang.org>
    Reviewed-by: Keith Randall <khr@golang.org>
    Reviewed-by: Cherry Mui <cherryyz@google.com>
    Run-TryBot: David Chase <drchase@google.com>