Allow comboboxes to invoke menus

[scottaohara]

closes #2050

This PR simply adds menu to the paragraph of text that indicates what value needs to be used for the aria-haspopup attribute, if the popup is not a listbox:

authors MUST specify an aria-haspopup value of tree, grid, or dialog that corresponds to the role of its popup.

I think some additional text could be written in the menu role definition to further acknowledge this new allowance. But I'll leave that open for discussion, rather than immediately start adding such text.

Test, Documentation and Implementation tracking

Once this PR has been reviewed and has consensus from the working group, tests should be written and issues should be opened on browsers. Add N/A and check when not applicable.

• [ ] "author MUST" tests:
• [ ] "user agent MUST" tests:
• [ ] Browser implementations (link to issue or commit):
  • WebKit:
  • Gecko:
  • Blink:
• [ ] Does this need AT implementations?
• [ ] Related APG Issue/PR:
• [ ] MDN Issue/PR:

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