Clarify how to model binary data in 3.1

This reorganizes binary data-related guidance into a
"Working With Binary Data" section, as has already been
done in 3.0.4.

This includes more detailed guidance on when various
approaches to binary data make sense (e.g. you cannot
stuff raw binary into JSON no matter what you put in your
Schema Object, and while you can base64-encode entire
message bodies, it takes up a lot more space for no clear
benefit).

Also note that only `multipart` media types with named parts
are supported, as they are modeled as an object.