Clarify menuitem[|radio|checkbox] menubar

[aardrian]

Closes https://github.com/w3c/aria/issues/2140

Explicitly notes in second paragraph description for each of menuitem, menuitemcheckbox and menuitemradio that they are allowed in menubar as well as menu. Makes consistent with Characteristics table for all 5 as well as final paragraph of description of first 3.

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