doc: mention KeepAlive & input args change

Change-Id: Icfb38f492ae71432858b7104fcba18a9ba835192
Reviewed-on: https://go-review.googlesource.com/29410Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.8.txt

@@ -12,6 +12,7 @@ Ports:
 
 API additions and behavior changes:
 
+cmd/compile: args no longer live until end of function - use runtime.KeepAlive instead (CL 28310)
 crypto/tls: set Conn.ConnectionState.ServerName unconditionally (CL 22862)
 crypto/tls: support AES-128-CBC cipher suites with SHA-256 (CL 27315)
 crypto/tls: don't generate random ticket keys if already set (CL 27317)