doc: mention that os package now uses poller

Updates #20587

Change-Id: I428d20bf8512ba859cc63c439bd5d92a5173fad2
Reviewed-on: https://go-review.googlesource.com/45910Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/go1.9.html

@@ -608,6 +608,15 @@ CL 40331: https://golang.org/cl/40331: cmd/link,runtime/cgo: enable PT_TLS gener
 </dl><!-- net/http/httptest -->
 
 <dl id="os"><dt><a href="/pkg/os/">os</a></dt>
+  <dd>
+    <p><!-- CL 36800 -->
+      The <code>os</code>package now uses the internal runtime poller
+      for file I/O.
+      This reduces the number of threads required for read/write
+      operations on pipes, and eliminates races when one goroutine
+      closes a file while another using it for I/O.
+    </p>
+
   <dd>
     <p><!-- CL 37915 -->
       On Windows,