bufio: add example for ReadFrom and remove unused code

Change-Id: Ia4fbb436ca573b1820f2b4d06d2332f588334768
Reviewed-on: https://go-review.googlesource.com/c/go/+/624357
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/bufio/bufio_test.go

@@ -939,7 +939,6 @@ func (t *testReader) Read(buf []byte) (n int, err error) {
 }
 
 func testReadLine(t *testing.T, input []byte) {
-	//for stride := 1; stride < len(input); stride++ {
 	for stride := 1; stride < 2; stride++ {
 		done := 0
 		reader := testReader{input, stride}

src/bufio/example_test.go

@@ -33,6 +33,33 @@ func ExampleWriter_AvailableBuffer() {
 	// Output: 1 2 3 4
 }
 
+// ExampleWriter_ReadFrom demonstrates how to use the ReadFrom method of Writer.
+func ExampleWriter_ReadFrom() {
+	var buf bytes.Buffer
+	writer := bufio.NewWriter(&buf)
+
+	data := "Hello, world!\nThis is a ReadFrom example."
+	reader := strings.NewReader(data)
+
+	n, err := writer.ReadFrom(reader)
+	if err != nil {
+		fmt.Println("ReadFrom Error:", err)
+		return
+	}
+
+	if err = writer.Flush(); err != nil {
+		fmt.Println("Flush Error:", err)
+		return
+	}
+
+	fmt.Println("Bytes written:", n)
+	fmt.Println("Buffer contents:", buf.String())
+	// Output:
+	// Bytes written: 41
+	// Buffer contents: Hello, world!
+	// This is a ReadFrom example.
+}
+
 // The simplest use of a Scanner, to read standard input as a set of lines.
 func ExampleScanner_lines() {
 	scanner := bufio.NewScanner(os.Stdin)