Scope and Decision Policy clarifications

A couple of changes based on feedback received by W3C Advisory Committee representatives during the internal review of the draft charter.

Changes made:
- The Decision Policy is now a copy and paste from that of the WebApps WG, and is thus explicit that decisions taken by the group will be compatible with asynchronous-only participation.
- The description of the 2UA case in the scope section is now more explicit that multiple controlling pages at the same time for one presentation session is possible.