design: remove vague clause from compatibility section
design: remove vague clause from compatibility section

Change-Id: I6fc2ebd570be36399e66869160f8bbbea364bcf0
Reviewed-on: https://go-review.googlesource.com/12887
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/proposal,golang/proposal