bytes: note that NewBuffer take ownership of its argument

Fixes #19383

Change-Id: Ic84517053ced7794006f6fc65e6f249e97d6cf35
Reviewed-on: https://go-review.googlesource.com/44691
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/bytes/buffer.go

@@ -441,10 +441,12 @@ func (b *Buffer) ReadString(delim byte) (line string, err error) {
 	return string(slice), err
 }
 
-// NewBuffer creates and initializes a new Buffer using buf as its initial
-// contents. It is intended to prepare a Buffer to read existing data. It
-// can also be used to size the internal buffer for writing. To do that,
-// buf should have the desired capacity but a length of zero.
+// NewBuffer creates and initializes a new Buffer using buf as its
+// initial contents. The new Buffer takes ownership of buf, and the
+// caller should not use buf after this call. NewBuffer is intended to
+// prepare a Buffer to read existing data. It can also be used to size
+// the internal buffer for writing. To do that, buf should have the
+// desired capacity but a length of zero.
 //
 // In most cases, new(Buffer) (or just declaring a Buffer variable) is
 // sufficient to initialize a Buffer.