design/25530-notary: expand rationale sections
design/25530-notary: expand rationale sections

Brad and I talked earlier and identified a few likely
misunderstandings about what I was trying to say
about having a single notary. Try to say it more clearly.

Also expand privacy section to include points and
observations from recent discussions on both golang-dev
and the issue tracker.

Change-Id: If89595c7bfc50255b6cba797ea1cf51437aa5ca0
Reviewed-on: https://go-review.googlesource.com/c/proposal/+/166197
Run-TryBot: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Reviewed-by: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

golang/proposal,golang/proposal