bufio.Scanner: delete obsolete TODO

Also fix the examples to use stderr for errors.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/7716052

src/pkg/bufio/example_test.go

@@ -19,7 +19,7 @@ func ExampleScanner_lines() {
 		fmt.Println(scanner.Text()) // Println will add back the final '\n'
 	}
 	if err := scanner.Err(); err != nil {
-		fmt.Fprintln(os.Stdout, "reading standard input:", err)
+		fmt.Fprintln(os.Stderr, "reading standard input:", err)
 	}
 }
 
@@ -37,7 +37,7 @@ func ExampleScanner_words() {
 		count++
 	}
 	if err := scanner.Err(); err != nil {
-		fmt.Fprintln(os.Stdout, "reading input:", err)
+		fmt.Fprintln(os.Stderr, "reading input:", err)
 	}
 	fmt.Printf("%d\n", count)
 	// Output: 15

src/pkg/bufio/scan.go

@@ -27,8 +27,6 @@ import (
 // control over error handling or large tokens, or must run sequential scans
 // on a reader, should use bufio.Reader instead.
 //
-// TODO(r): Provide executable examples.
-//
 type Scanner struct {
 	r            io.Reader // The reader provided by the client.
 	split        SplitFunc // The function to split the tokens.