Easier selection of a primary document

Came from a client.

Considerations

• should be a simpler way, than is currently available, to select a primary document for a page
• what if a user selects a primary document for a page, but the page doesn't use it at all?
• should we highlight to a user when they are editing a document that is a primary document for a page?
• should there be, somewhere, a 'Make primary document' button, and where in the UI/flow should this be?
• should we highlight to a user if a page doesn't yet have a primary document assigned? should we automatically select a primary document for that page?
• should we highlight when multiple pages use the same primary document? what problems does this cause?
• corner case: 2 banners on two landing-pages, and you want to control to which page the banners link to. Using the primary page document you can control this for one of the two banners by making it the primary page document. However, for the other banner it will use the highest sitemap-item. It would be nice if you could control this for both of the documents (more than just one document on the page, e.g. make the primary document a multiple).