add REST documentation for content module

If I understand correctly, the endpoint names are only used by the parser and must be unique per endpoint. Other that the uniqueness requirement, though, they're completely arbitrary, although I do think they show up as the 'nickname' property in the raw Swagger data.

The approach that seems easiest to maintain (to me) is to concatenate the method with a camelCase version of the path. That _should_ guarantee uniqueness, and it should make it easy for a developer that's documenting a new endpoint to define a nickname without worrying about a potential naming conflict.

If we start "shortening" the path in some cases, developers will have to search though the source code to ensure uniqueness of their names. That could be especially problematic since, in some cases (e.g. principals or authentication) the various endpoints are scattered across multiple files. And I don't think we can create a generally applicable rule that will result in short nicknames universally. For example, in the activities module, there is both a `/api/activity` and a `/api/activity/:activityStreamId`endpoint.