Add migration to WebAssembly information to PNaCl / NaCl docs.

native_client_sdk/doc_generated/devguide/devcycle/building.html

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-<section id="building">
-<span id="devcycle-building"></span><h1 id="building"><span id="devcycle-building"></span>Building</h1>
+<b><font color="#cc0000">
+NOTE:
+Deprecation of the technologies described here has been announced
+for platforms other than ChromeOS.<br/>
+Please visit our
+<a href="/native-client/migration">migration guide</a>
+for details.
+</font></b>
+<hr/><section id="building">
+<h1 id="building">Building</h1>
 <div class="contents local" id="table-of-contents" style="display: none">
 <p class="topic-title first">Table Of Contents</p>
 <ul class="small-gap">
 <li><p class="first"><a class="reference internal" href="#introduction" id="id4">Introduction</a></p>
 <ul class="small-gap">
 <li><a class="reference internal" href="#target-architectures" id="id5">Target architectures</a></li>
 <li><a class="reference internal" href="#c-libraries" id="id6">C libraries</a></li>
 <li><a class="reference internal" href="#c-standard-libraries" id="id7">C++ standard libraries</a></li>
 <li><a class="reference internal" href="#sdk-toolchains" id="id8">SDK toolchains</a></li>
 <li><a class="reference internal" href="#sdk-toolchains-versus-your-hosted-toolchain" id="id9">SDK toolchains versus your hosted toolchain</a></li>
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 Function foo has disallowed type: i128 (i128)
 LLVM ERROR: PNaCl ABI verification failed
 </pre>
 <p>When faced with a PNaCl ABI verification error, check the list of features
 that are <a class="reference internal" href="/native-client/nacl-and-pnacl.html#when-to-use-nacl"><em>not supported by PNaCl</em></a>.
 If the problem you face is not listed as restricted,
 <a class="reference internal" href="/native-client/help.html#help"><em>let us know</em></a>!</p>
 </section>
 
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