bufio: document relationship between UnreadByte/UnreadRune and Peek

Fixes #29387

Change-Id: I2d9981f63ac16630ed39d6da6692c81396f4e9ea
Reviewed-on: https://go-review.googlesource.com/c/155930
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/bufio/bufio.go

@@ -33,8 +33,8 @@ type Reader struct {
 	rd           io.Reader // reader provided by the client
 	r, w         int       // buf read and write positions
 	err          error
-	lastByte     int
-	lastRuneSize int
+	lastByte     int // last byte read for UnreadByte; -1 means invalid
+	lastRuneSize int // size of last rune read for UnreadRune; -1 means invalid
 }
 
 const minReadBufferSize = 16
@@ -123,6 +123,9 @@ func (b *Reader) readErr() error {
 // being valid at the next read call. If Peek returns fewer than n bytes, it
 // also returns an error explaining why the read is short. The error is
 // ErrBufferFull if n is larger than b's buffer size.
+//
+// Calling Peek prevents a UnreadByte or UnreadRune call from succeeding
+// until the next read operation.
 func (b *Reader) Peek(n int) ([]byte, error) {
 	if n < 0 {
 		return nil, ErrNegativeCount
@@ -252,6 +255,10 @@ func (b *Reader) ReadByte() (byte, error) {
 }
 
 // UnreadByte unreads the last byte. Only the most recently read byte can be unread.
+//
+// UnreadByte returns an error if the most recent method called on the
+// Reader was not a read operation. Notably, Peek is not considered a
+// read operation.
 func (b *Reader) UnreadByte() error {
 	if b.lastByte < 0 || b.r == 0 && b.w > 0 {
 		return ErrInvalidUnreadByte
@@ -290,8 +297,8 @@ func (b *Reader) ReadRune() (r rune, size int, err error) {
 	return r, size, nil
 }
 
-// UnreadRune unreads the last rune. If the most recent read operation on
-// the buffer was not a ReadRune, UnreadRune returns an error.  (In this
+// UnreadRune unreads the last rune. If the most recent method called on
+// the Reader was not a ReadRune, UnreadRune returns an error. (In this
 // regard it is stricter than UnreadByte, which will unread the last byte
 // from any read operation.)
 func (b *Reader) UnreadRune() error {