Change combobox role. (#1051)

See https://github.com/w3c/aria/wiki/Resolving-ARIA-1.1-Combobox-Issues for full description of changes and motivations

* Revise description of user agent responsibilities for aria-activedescendant
* Remove combobox from list of composites for which authors must manage focus in section with id managingfocus_authors
* Revise prose of section managingfocus_authors to remove implications that the active descendant is always a DOM descendant of the element with DOM focus.
* Revise combobox example in aria-activedescendant definition so it does not describe textbox as contained by a combobox
* Add editor's note
* Revise aria-autocomplete description to match new combobox structure
* Add extra combobox warning