webscreens charter update

Pull request with changes discussed on-list.  Summary since it's a long patch:

(1) Added a section to the first paragraph clarifying potential display discoverability scenarios - wired, wireless, Internet/cloud based.

(2) Updated text throughout to refer to "secondary displays" (plural) to include one-app-to-multiple display scenarios.

(3) Updated text to refer to the controlling entity as a "web application" generally, consisting of "authorized pages" that may be hosted on multiple UAs (when content is rendered remotely).

(4) Moved discussion of standardization of the network level protocols for wireless content sharing to the "out of scope" section.

(5) Updated text describing content to be shown as "web content" generically, with explicit mention of HTML documents, media types, and application-specific media.