io: add examples for (*SectionReader) Read/Size

Change-Id: Ie804f9a5d1d6beec210d2f8075c030a5c60a78ea
Reviewed-on: https://go-review.googlesource.com/c/go/+/352152
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Heschi Kreinick <heschi@google.com>

src/io/example_test.go

@@ -160,6 +160,21 @@ func ExampleSectionReader() {
 	// io.Reader stream
 }
 
+func ExampleSectionReader_Read() {
+	r := strings.NewReader("some io.Reader stream to be read\n")
+	s := io.NewSectionReader(r, 5, 17)
+
+	buf := make([]byte, 9)
+	if _, err := s.Read(buf); err != nil {
+		log.Fatal(err)
+	}
+
+	fmt.Printf("%s\n", buf)
+
+	// Output:
+	// io.Reader
+}
+
 func ExampleSectionReader_ReadAt() {
 	r := strings.NewReader("some io.Reader stream to be read\n")
 	s := io.NewSectionReader(r, 5, 17)
@@ -191,6 +206,16 @@ func ExampleSectionReader_Seek() {
 	// stream
 }
 
+func ExampleSectionReader_Size() {
+	r := strings.NewReader("some io.Reader stream to be read\n")
+	s := io.NewSectionReader(r, 5, 17)
+
+	fmt.Println(s.Size())
+
+	// Output:
+	// 17
+}
+
 func ExampleSeeker_Seek() {
 	r := strings.NewReader("some io.Reader stream to be read\n")