net, doc: document JoinHostPort change in release notes, clarify its docs

Updates #18059
Updates #20587

Change-Id: Icbb4c7cb201ac51d2cc6066620b47ba09ff6fe65
Reviewed-on: https://go-review.googlesource.com/45780Reviewed-by: Chris Broadfoot <cbro@golang.org>

doc/go1.9.html

@@ -466,12 +466,9 @@ type T1 = T2
       TODO: <a href="https://golang.org/cl/37260">https://golang.org/cl/37260</a>: allow Resolver to use a custom dialer
     </p>
 
-    <p><!-- CL 37402 -->
-      TODO: <a href="https://golang.org/cl/37402">https://golang.org/cl/37402</a>: implement deadline functionality on Pipe
-    </p>
-
     <p><!-- CL 40510 -->
-      TODO: <a href="https://golang.org/cl/40510">https://golang.org/cl/40510</a>: don&#39;t enclose non-literal IPv6 addresses in square brackets
+      <a href="/pkg/net/#JoinHostPort"><code>JoinHostPort</code></a> now only places an address in square brackets if the host contains a colon.
+      In previous releases it would also wrap addresses in square brackets if they contained a percent ('<code>%</code>') sign.
     </p>
 
     <p><!-- CL 40512 -->

src/net/ipsock.go

@@ -194,8 +194,8 @@ func splitHostZone(s string) (host, zone string) {
 }
 
 // JoinHostPort combines host and port into a network address of the
-// form "host:port" or "host%zone:port", if host is a literal IPv6
-// address, "[host]:port" or [host%zone]:port.
+// form "host:port". If host contains a colon, as found in literal
+// IPv6 addresses, then JoinHostPort returns "[host]:port".
 func JoinHostPort(host, port string) string {
 	// We assume that host is a literal IPv6 address if host has
 	// colons.