handle ins/del in header return types

[bakkot]

From #609:

<emu-clause id="sec-foobar" type="abstract operation">
  <h1>
    FooBar (
      _v_: an ECMAScript language value,
    ): <ins>an integer</ins><del>either a normal completion containing an integer, or a throw completion</del>
  </h1>
  <dl class="header">
    <dt>description</dt>
    <dd></dd>
  </dl>
  <emu-alg>
    1. <del>Throw a *RangeError* exception.</del>
    1. Return 42.
  </emu-alg>
</emu-clause>

errors with

(file):5:23: unknown extra text in header