[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?