docs: Introduce cascading style <div> and <p> continued on <h[2-5]>.

<h2>Section Example</h2>
<div> <!-- h2+div is applied -->
<p>Section preamble.</p>

<h3>Subsection Example</h3>
<p> <!-- h3+p is applied -->
Subsection body
</p>

<!-- End of section body -->
</div>

FIXME: Care H5 better.
llvm-svn: 130040