Document pagination behavior for query-based endpoints

[jpahm]

Currently, the paginated nature of the query-based endpoints (i.e. /course, /course/sections) is not explicitly documented in the OpenAPI spec, which is a considerable flaw. We should add the offset query parameter to the OpenAPI doc comments (via @param) on the relevant controllers and properly document its behavior. The details of how the pagination behaves and the usage of the offset parameter should be referenced in the description of the endpoint as well.

[justinschwerdtfeger]

I would like to work on this issue