ACTION-1490: Modify definition of aria-activedesendant

Hi Matt,

Based on last week's ARIA call, there is an action to modify the 
definition of aria-activedescendant to handle the case of a combobox 
where focus is on its textbox, and that textbox is using 
aria-activedescendant to reference the active listbox option.  I've made 
a first pass at it by adding a paragraph to the definition:

"
A special use case is a combobox where its textbox has DOM focus, and 
that textbox employs an aria-activedescendant attribute to reference the 
currently active listbox option. The referenced listbox option is not in 
fact a descendant of the textbox itself, but of its parent combobox 
element. Even so, the aria-activedescendant property is treated by user 
agents as the active descendant of the combobox, and not the textbox.
"

Here's a link to the modified definition on rawgit:
http://rawgit.com/w3c/aria/mck-action1490/aria/aria.html#aria-activedescendant

-- 
;;;;joseph.

'Die Wahrheit ist Irgendwo da Draußen. Wieder.'
                  - C. Carter -