Some constants were added above the initial copyright blurb, and then
later a new copyright blurb was added on top of that. So we wound up
with two header sections, one of which contained a useful comment that
became obscured. This commit fixes up that mistake.

Change-Id: I8b9b8c34495cdceae959e151e8ccdee3137f6ca4
Reviewed-on: https://go-review.googlesource.com/c/go/+/191841
Run-TryBot: Jason A. Donenfeld <Jason@zx2c4.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>