Make site_api's documentation more generic

[ChrisLovering]

site_api was originally made to wrap our django site api. However, with SirRobin, it also wraps the codejam management system, with no changes required.

This is because the implementation is generic enough that is just works.

As such we should update the docs around the site_api to make them a little more generic, describing what it can and can't be used for.