Add more extensive section on Client media selection including margin discussion

[wifiprintguy]

There's not much discussion about how Clients are supposed to act on various Printer Description attributes. (This gets back to the "feature" discussion we've been slowly having in the IPP WG.) How a Client selects media and media variants should be covered a bit and then the reader can be referred to MSN 2.1, JOBEXT, and so forth.

[michaelrsweet]

Started some work on this:

[master 4b9bb7f] Update document references and add initial documentation of media and media-col (Issue #3)