Additional UI guidelines for user agents

[markafoltz]

This PR addresses two issues:

• Implementation guideline: users should have a way to know which origin is being presented at any time  #223: Implementation guideline: users should have a way to know which origin is being presented at any time
• Think more about privacy implications in multi-user scenarios #224: Think more about privacy implications in multi-user scenarios

It adds a guideline that controlling user agents should display the origin of the URL that is to be presented. It also adds guidelines about notifying the user when there are multiple parties that can control of a presentation.