init_facets: Add documentation
init_facets is not included in the default sphinx documentation but this
docstring should help people digging into cubicweb core. This could go in an
Advanced developer section of the book.
<div id="ajaxroot">
<div class="ajaxHtmlHead">
<script src="http://foo.js" type="text/javascript"> </script>
<link rel="stylesheet" type="text/css" media="all" href="qunit.css" />
</div>
<h1>Hello</h1>
</div>