doc: note GCM behaviour change in Go 1.6.

This change documents the behaviour change caused by
https://go-review.googlesource.com/18480 in Go 1.6.

Updates #13886

Change-Id: I2daa08a62775bbc209f0f4cbeae21b8184ce7609
Reviewed-on: https://go-review.googlesource.com/18481Reviewed-by: Russ Cox <rsc@golang.org>

doc/go1.6.html

@@ -514,6 +514,12 @@ now report
 <a href="/pkg/io/#EOF"><code>io.EOF</code></a>.
 </li>
 
+<li>
+The <a href="/pkg/crypto/cipher/"><code>crypto/cipher</code></a> package now
+overwrites the destination buffer in the event of a GCM decryption failure.
+This is to allow the AESNI code to avoid using a temporary buffer.
+</li>
+
 <li>
 The <a href="/pkg/crypto/tls/"><code>crypto/tls</code></a> package
 has a variety of minor changes.