This change implements the convention for generated code header agreed upon in https://golang.org/s/generatedcode.
Additionally run go generate.
Also update some comments.

Updates #13560

Change-Id: If45f91b93aaa0d43280c2c4630823bc4d2dc7d3a
Reviewed-on: https://go-review.googlesource.com/60250
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>