Use generated code comments following the convention
https://golang.org/s/generatedcode.

Follow-up for CL 204659

Change-Id: I7a3467ff9477488616b17f059245368d3d82d7a0
Reviewed-on: https://go-review.googlesource.com/c/go/+/208417
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>