Schema & Hypermedia follow ups

[tomchristie]

Placeholder for now to ensure that anything from #4179 that doesn't make the 3.4.0 cut can be assigned to this ticket.

Currently:

• Add descriptions to schema generation, and document use of describe.
• Supporting id as a keyword argument over pk.
• Adding type annotations to fields (including in coreapi spec)
• Handle binary downloads in coreapi library.
• Document file uploads and downloads in API clients docs.
• Add to quickstart?
• Document custom codecs.

[robermorales]

"Adding type annotations to fields" seems to be the needed feature to make swagger ui display properly the parameters types. When it will be available? Can I help in some way?

[tomchristie]

Already well underway in the existing 3.5 branch. Will be available later this month.

[tomchristie]

Folding remaining points into #4549 and #4555