bytes: mention strings.Builder in Buffer.String docs

Fixes #22778

Change-Id: I37f7a59c15828aa720fe787fff42fb3ef17729c7
Reviewed-on: https://go-review.googlesource.com/80815
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/bytes/buffer.go

@@ -56,6 +56,8 @@ func (b *Buffer) Bytes() []byte { return b.buf[b.off:] }
 
 // String returns the contents of the unread portion of the buffer
 // as a string. If the Buffer is a nil pointer, it returns "<nil>".
+//
+// To build strings more efficiently, see the strings.Builder type.
 func (b *Buffer) String() string {
 	if b == nil {
 		// Special case, useful in debugging.