doc: mention os/exec StdinPipe change in Go 1.2 doc

Fixes #6439.

R=r, minux.ma
CC=golang-dev
https://golang.org/cl/13478045

doc/go1.2.html

@@ -823,6 +823,17 @@ are absorbed by the
 and the client receives an empty body as required by the HTTP specification.
 </li>
 
+<li>
+The <a href="/pkg/os/exec/"><code>os/exec</code></a> package's 
+<a href="/pkg/os/exec/#Cmd.StdinPipe"><code>Cmd.StdinPipe</code></a> method 
+returns an <code>io.WriteCloser</code>, but has changed its concrete
+implementation from <code>*os.File</code> to an unexported type that embeds
+<code>*os.File</code>, and it is now safe to close the returned value.
+Before Go 1.2, there was an unavoidable race that this change fixes.
+Code that needs access to the methods of <code>*os.File</code> can use an
+interface type assertion, such as <code>wc.(interface{ Sync() error })</code>.
+</li>
+
 <li>
 The <a href="/pkg/runtime/"><code>runtime</code></a> package relaxes
 the constraints on finalizer functions in