doc: note quoted cookie value change in Go 1.9 relnotes

Fixes #20834.

Change-Id: Ic60000db6b8ae489a5cedbdc938d9ca1dc34c6d4
Reviewed-on: https://go-review.googlesource.com/47151Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/go1.9.html

@@ -580,6 +580,12 @@ CL 40331: https://golang.org/cl/40331: cmd/link,runtime/cgo: enable PT_TLS gener
 <dl id="net/http"><dt><a href="/pkg/net/http/">net/http</a></dt>
   <dd>
 
+    <p><!-- CL 37328 -->
+      The <a href="/pkg/net/http/#Cookie.String"><code>Cookie.String</code></a> method, used for
+      <code>Cookie</code> and <code>Set-Cookie</code> headers, now encloses values in double quotes
+      if the value contains either a space or a comma.
+    </p>
+
     <p>Server changes:</p>
     <ul>
       <li><!-- CL 38194 -->