doc/go1.5.txt: bytes, strings: add LastIndexByte

Change-Id: I05cfacd746e87011de8b659ab3b2fbe23146a7f3
Reviewed-on: https://go-review.googlesource.com/9504Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

doc/go1.5.txt

@@ -26,6 +26,7 @@ archive/zip: add (*Writer).SetOffset method (https://golang.org/cl/7445)
 bufio: add Reader.Discard (https://golang.org/cl/2260)
 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/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)