cmd/go: document convention to signify generated code.

This change updates the go tool's documentation under the section
"Generate Go files by processing source" to mention the convention that
generated source files should have a line of text that matches the
following regular expression:

    ^// Code generated .* DO NOT EDIT\.$

Previously, the canonical documentation for this convention
(https://golang.org/s/generatedcode) referenced Rob Pike's comment at
https://golang.org/issue/13560#issuecomment-288457920. This change
merely moves that information to a more visible place.

Updates #25433.

Change-Id: I804d95d307d1dc68cb28da3750ebe9090178c474
Reviewed-on: https://go-review.googlesource.com/118756


Reviewed-by: Rob Pike <r@golang.org>