Draft guidance section about properties that describe widget features or behaviors

[mcking65]

This section will clarify the meaning and purpose of the following attributes:

• [ ] aria-autocomplete
• [ ] aria-controls
• [ ] aria-haspopup
• [ ] aria-keyshortcuts
• [ ] aria-multiline
• [ ] aria-multiselectable
• [ ] aria-orientation

Note that it is related to issue #77 about aria-haspopup.