Header elements (h1-h6) cannot be placed inside <summary>

[akazukin5151]

The spec does say it's for Phrasing contents (which excludes header elements), but then allows them anyway.

Content model:
Phrasing content, optionally intermixed with heading content.

https://html.spec.whatwg.org/multipage/interactive-elements.html#the-summary-element

https://stackoverflow.com/questions/41227862/is-it-semantically-correct-to-use-h2-tag-inside-summary-tag

So the compile error for this code, does not follow the spec

let x: DOMTree<String> = html! {
    <summary>
      <h4>"test"</h4>
    </summary>
};

error[E0277]: the trait bound `h4<_>: axohtml::elements::PhrasingContent<_>` is not satisfied
  --> src/main.rs:9:30
   |
9  |       let x: DOMTree<String> = html! {
   |  ______________________________^
10 | |         <summary>
11 | |           <h4>"test"</h4>
12 | |         </summary>
13 | |     };
   | |_____^ the trait `axohtml::elements::PhrasingContent<_>` is not implemented for `h4<_>`
   |

This feels gnarly to carve out an exception in our type system, so I understand if this isn't high priority. It's not high priority for me either