spec: clarifications based on feedback

This change includes several smaller changes based on feedback
received so far.

These changes were reviewed at CL 385536. The only additional
change here is to the current date in the subtitle.

Change-Id: I653eb4a143e3b86c5357a2fd3b19168419c9f432
Reviewed-on: https://go-review.googlesource.com/c/go/+/390634


Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>