text/scanner: add runnable example for package

Change-Id: I42a952b04a56fb888fa7d5d9c2b56cbdd3434034
Reviewed-on: https://go-review.googlesource.com/11246Reviewed-by: Andrew Gerrand <adg@golang.org>

src/text/scanner/example_test.go

+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package scanner_test
+
+import (
+	"fmt"
+	"strings"
+	"text/scanner"
+)
+
+func Example() {
+	const src = `
+	// This is scanned code.
+	if a > 10 {
+		someParsable = text
+	}`
+	var s scanner.Scanner
+	s.Init(strings.NewReader(src))
+	var tok rune
+	for tok != scanner.EOF {
+		tok = s.Scan()
+		fmt.Println("At position", s.Pos(), ":", s.TokenText())
+	}
+
+	// Output:
+	// At position 3:4 : if
+	// At position 3:6 : a
+	// At position 3:8 : >
+	// At position 3:11 : 10
+	// At position 3:13 : {
+	// At position 4:15 : someParsable
+	// At position 4:17 : =
+	// At position 4:22 : text
+	// At position 5:3 : }
+	// At position 5:3 :
+}

src/text/scanner/scanner.go

@@ -12,17 +12,6 @@
 // literals as defined by the Go language specification.  It may be
 // customized to recognize only a subset of those literals and to recognize
 // different identifier and white space characters.
-//
-// Basic usage pattern:
-//
-//	var s scanner.Scanner
-//	s.Init(src)
-//	tok := s.Scan()
-//	for tok != scanner.EOF {
-//		// do something with tok
-//		tok = s.Scan()
-//	}
-//
 package scanner
 
 import (