io: add doc/next file and remove errors.Is from example

Author: grishy

doc/next/6-stdlib/99-minor/io/51115.md

@@ -0,0 +1,4 @@
+<!-- go.dev/issue/51115 -->
+The new [LimitedReader.Err] field allows returning a custom error
+when the read limit is exceeded. When nil (the default), [LimitedReader.Read]
+returns [EOF] when the limit is reached, maintaining backward compatibility.

src/io/example_test.go

@@ -132,17 +132,15 @@ func ExampleLimitedReader_Err() {
 	if err != nil {
 		log.Fatal(err)
 	}
-	fmt.Printf("read %d bytes: %q\n", n, buf[:n])
+	fmt.Printf("%d; %q\n", n, buf[:n])
 
 	// try to read more and get the custom error
 	n, err = lr.Read(buf)
-	if errors.Is(err, sentinel) {
-		fmt.Println("error:", err)
-	}
+	fmt.Printf("%d; error: %v\n", n, err)
 
 	// Output:
-	// read 4 bytes: "some"
-	// error: read limit reached
+	// 4; "some"
+	// 0; error: read limit reached
 }
 
 func ExampleMultiReader() {

src/io/io.go

@@ -556,10 +556,8 @@ func (s *SectionReader) Read(p []byte) (n int, err error) {
 	return
 }
 
-var (
-	errWhence = errors.New("Seek: invalid whence")
-	errOffset = errors.New("Seek: invalid offset")
-)
+var errWhence = errors.New("Seek: invalid whence")
+var errOffset = errors.New("Seek: invalid offset")
 
 func (s *SectionReader) Seek(offset int64, whence int) (int64, error) {
 	switch whence {