doc: add net/smtp TLSConnectionState note in go1.5.txt

Change-Id: Ia18f6ed0fa38dac297fc5fe1ca56e38ec37518ef
Reviewed-on: https://go-review.googlesource.com/2152Reviewed-by: Brad Fitzpatrick <bradfitz@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)
+net/smtp: add TLSConnectionState accessor (https://golang.org/cl/2151)