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/118756Reviewed-by: Rob Pike <r@golang.org>

src/cmd/go/alldocs.go

@@ -419,6 +419,12 @@
 // (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 early in the file that
+// matches the following regular expression (in Go syntax):
+//
+// 	^// Code generated .* DO NOT EDIT\.$
+//
 // 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

@@ -47,6 +47,12 @@ 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 early in the file that
+matches the following regular expression (in Go syntax):
+
+	^// Code generated .* DO NOT EDIT\.$
+
 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.