[menu] Explicitly mention support for HR as a separator

[lukewarlow]

It's common to visually separate menu items and select supports this so it seems good consistent for menu to do this too?

[scottaohara]

"good"

tbh, i the inclusion of hr in select serves more as a visual indicator of grouping than anything actually meaningful to people who can't see. in either case (now with customizable select) it'd be better, imo, to actually group the elements you want grouped using optgroup or fieldset, and then add whatever visual separator you want.

but, if we want to make this actually useful to everyone - if implicit groupings could be made in the a11y tree (for either element) then I wouldn't have to write all this stuff and I could have just thumbs upped your response. me shutting up is really the end goal, isn't it?

[github-actions]

There hasn't been any discussion on this issue for a while, so we're marking it as stale. If you choose to kick off the discussion again, we'll remove the 'stale' label.

[domfarolino]

Since our proposal indeed already supports <fieldset> as a grouping element, I think that leaves us with "and then add whatever visual separator you want", which I think using <hr> is good for. So I'm hopeful that people are happy with us supporting both <fieldset> and <hr> inside menu lists. That's what #1324 clarifies, and what we've been assuming in our prototype, so I'll mark this as will-by-fixed by #1324, the explainer rewrite.