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>
34 files changed