Collecting requirements for User Contexts

At the meeting today I volunteered to collect all requirements which have been
suggested for inclusion in the User Contexts work, encompassing, specifically,
both of the documents submitted by Andy and Rich and the spec as currently
drafted. I have also drawn on my memory of discussions at our meetings, while
applying some editorial judgment to all of the above. I don't think I've
captured everything yet (omissions are not intentional, needless to say, but
rather the result of trying to combine material from multiple sources).
Editorial notes are liberally included.

Corrections, clarifications and additions are welcome. Exclusions and
discussion of scope should be postponed for now. I've noted the major issues.

A draft is attached. I'm trying to follow MediaWiki markup but I may have made
mistakes in that regard as I don't write for wikis frequently.