design: tweaks to 14313
design: tweaks to 14313

Clarify and fix various things based on comments from issue
go/golang#14313.

Change-Id: I53a7005b2b6c5c0e973373febfae5f8278d867bc
Reviewed-on: https://go-review.googlesource.com/21030
Reviewed-by: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

golang/proposal,golang/proposal