cmd/go: fix go generate docs

The docs were never updated for the change to the placement
of the DO NOT EDIT line.

Also, the description of the DO NOT EDIT line interrupted the
description of the //go:generate line, which made for some
confusing references in the text that followed. Move it lower.

Fixes #41196.

Change-Id: I6af2a199fa98d45f5ccac7cdf7e9e54257699e61
Reviewed-on: https://go-review.googlesource.com/c/go/+/283633


Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>