doc: mention the crypto/cipher.Stream clarification in go1.5.txt

Change-Id: Ie95c86d222b230a2e69f4141e3372bf94dee3e91
Reviewed-on: https://go-review.googlesource.com/2129Reviewed-by: Minux Ma <minux@golang.org>

doc/go1.5.txt

+crypto/cipher: clarify what will happen if len(src) != len(dst) for the Stream interface. (https://golang.org/cl/1754)
 crypto/tls: change default minimum version to TLS 1.0. (https://golang.org/cl/1791)
 encoding/base64: add unpadded encodings (https://golang.org/cl/1511)