[UR] Expand Loader API documentation with a section in the programming guide.

[aarongreig]

The programming guide contains some explanatory notes about UR objects and API usage. It predates the introduction of the loader level APIs so there isn't a section in there for them. We should write one, and specifically we should include a clarification of the expected behaviour if you do urLoaderInit -> urLoaderTeardown -> urLoaderInit (nothing good I'm sure, but we should write that down)