doc/go1.5.txt: mention that 1.5 supports non-standard GCM lengths.

Change-Id: I7cad3f7af2452e29b3dae3da87cbd24013f6dae6
Reviewed-on: https://go-review.googlesource.com/10850Reviewed-by: Adam Langley <agl@golang.org>

doc/go1.5.txt

@@ -35,6 +35,7 @@ bytes: add Buffer.Cap (https://golang.org/cl/8342)
 bytes, strings: add Reader.Size (https://golang.org/cl/3199)
 bytes, strings: add LastIndexByte (https://golang.org/cl/9500)
 crypto/cipher: clarify what will happen if len(src) != len(dst) for the Stream interface. (https://golang.org/cl/1754)
+crypto/cipher: support non-standard nonce lengths for GCM. (https://golang.org/cl/8946)
 crypto/elliptic: add Name field to CurveParams struct (https://golang.org/cl/2133)
 crypto/elliptic: Unmarshaling points now automatically checks that the point is on the curve (https://golang.org/cl/2421)
 crypto/tls: change default minimum version to TLS 1.0. (https://golang.org/cl/1791)