internal/gen: use new DO NOT EDIT comment standard. See proposal golang/go#13560. Also updated gen.go to include all packages using internal/gen and to update the “Copy” comment. Change-Id: Iad084642af3700a112b4305aef0c55b475893106 Reviewed-on: https://go-review.googlesource.com/38368 Run-TryBot: Marcel van Lohuizen <mpvl@golang.org> Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>