Revert "bytes: document that Buffer values must not be copied"

This reverts commit b1177d39.

Change-Id: Iffbd63d5993633143a81572b223e3c5dd2353206
Reviewed-on: https://go-review.googlesource.com/12581Reviewed-by: Alan Donovan <adonovan@google.com>

src/bytes/buffer.go

@@ -14,7 +14,6 @@ import (
 
 // A Buffer is a variable-sized buffer of bytes with Read and Write methods.
 // The zero value for Buffer is an empty buffer ready to use.
-// Buffer values must not be copied.
 type Buffer struct {
 	buf       []byte            // contents are the bytes buf[off : len(buf)]
 	off       int               // read at &buf[off], write at &buf[len(buf)]