doc: update overview for authentication
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 <e321a8fd89c4908465ddaf8d5d0d62480efacb63@golang.org>
Run-TryBot: Katie Hockman <e321a8fd89c4908465ddaf8d5d0d62480efacb63@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/go,CAFxX/go,CAFxX/go,cloudflare/go,golang/go,golang/go,cloudflare/go,nawawi/go,nawawi/go,nawawi/go,golang/go,nawawi/go,CAFxX/go,golang/go,CAFxX/go,CAFxX/go,nawawi/go,cloudflare/go,golang/go,CAFxX/go,nawawi/go,nawawi/go,cloudflare/go,CAFxX/go,cloudflare/go,cloudflare/go,golang/go,cloudflare/go,cloudflare/go,CAFxX/go,nawawi/go,golang/go