cmd/go: copy missing bit of documentation about code generated comment

This CL attempts to restore the clarity of the original specification
at https://golang.org/s/generatedcode that the line may appear
anywhere. It is preferable (for human readability), and most common
for it to be early in the file, but that is merely a convention, not
a strict well-specified requirement. Document it as so.

Background

Issue #13560 was a proposal define a standard for marking files as
generated, one that is suitable to be recognized both by humans
and machine tools. It was accepted, and the final specification
was documented at https://golang.org/s/generatedcode. Its text,
copied exactly:

	Generated files are marked by a line of text that matches
	the regular expression, in Go syntax:

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

	The .* means the tool can put whatever folderol it wants in there,
	but the comment must be a single line and must start with Code generated
	and end with DO NOT EDIT., with a period.

	The text may appear anywhere in the file.

The https://golang.org/s/generatedcode link points to a comment
in a very large GitHub issue. That makes it harder to find.
Issue #25433 was opened about moving that information somewhere else.
It was resolved via CL 118756, which added text to cmd/go documentation
at https://golang.org/cmd/go/#hdr-Generate_Go_files_by_processing_source:

	To convey to humans and machine tools that code is generated,
	generated source should have a line early in the file that
	matches the following regular expression (in Go syntax):

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

The CL description noted that "This change merely moves that
information to a more visible place." The intention was to preserve
the specification unmodified.

The original specification was very clear that "The text may appear
anywhere in the file." The new text in cmd/go documentation wasn't
very clear. "A line early in the file" is not a precise enough criteria
to be recognized by a machine tool, because there isn't a precise
definition of what lines are "early in the file".

Updates #13560
Updates #25433
Updates #28089

Change-Id: I4e374163b16c3f972f9591ec2647fd3d5a2dd5ae
Reviewed-on: https://go-review.googlesource.com/c/158817Reviewed-by: Rob Pike <r@golang.org>

src/cmd/go/alldocs.go

@@ -442,11 +442,14 @@
 // command alias, described below.
 //
 // To convey to humans and machine tools that code is generated,
-// generated source should have a line early in the file that
-// matches the following regular expression (in Go syntax):
+// 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.

src/cmd/go/internal/generate/generate.go

@@ -49,11 +49,14 @@ that can be run locally. It must either be in the shell path
 command alias, described below.
 
 To convey to humans and machine tools that code is generated,
-generated source should have a line early in the file that
-matches the following regular expression (in Go syntax):
+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.