doc: update overview for authentication
    
    The instructions have already been updated in greater
    detail in "Step 2: Configure git authentication", but
    the overview needs updated to reflect the new workflow.
    
    Change-Id: I6f411a3dc500a9058036a4a828403c0153e4220a
    Reviewed-on: https://go-review.googlesource.com/c/go/+/256857
    Trust: Katie Hockman <katie@golang.org>
    Run-TryBot: Katie Hockman <katie@golang.org>
    Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
    TryBot-Result: Go Bot <gobot@golang.org>