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 <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: rsc

src/cmd/go/alldocs.go

@@ -52,15 +52,6 @@ that can be run locally. It must either be in the shell path
 (gofmt), a fully qualified path (/usr/you/bin/mytool), or a
 command alias, described below.
 
-To convey to humans and machine tools that code is generated,
-generated source should have a line that matches the following
-regular expression (in Go syntax):
-
-	^// Code generated .* DO NOT EDIT\.$
-
-The line may appear anywhere in the file, but is typically
-placed near the beginning so it is easy to find.
-
 Note that go generate does not parse the file, so lines that look
 like directives in comments or multiline strings will be treated
 as directives.
@@ -72,6 +63,15 @@ arguments when it is run.
 Quoted strings use Go syntax and are evaluated before execution; a
 quoted string appears as a single argument to the generator.
 
+To convey to humans and machine tools that code is generated,
+generated source should have a line that matches the following
+regular expression (in Go syntax):
+
+	^// Code generated .* DO NOT EDIT\.$
+
+This line must appear before the first non-comment, non-blank
+text in the file.
+
 Go generate sets several variables when it runs the generator:
 
 	$GOARCH