design: clarify CoC point about unconstructive criticism
design: clarify CoC point about unconstructive criticism

Update golang/go#13073

Change-Id: Icb07fc49c854342569599e4bb723ade55593d8c9
Reviewed-on: https://go-review.googlesource.com/17018
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/proposal,golang/proposal