add REST documentation for content module

No, this should not have been converted back to a `body` parameter. Using FormParams gives a nice little form that allows you to create a new content item. You can even enter an individual manager and an individual member. The only limitation is that it's not possible to enter an array of managers or an array of members, but once Swagger UI adds support for array form params, that'll start working as well.

The only thing we need to make sure is that the action that is being described (e.g. creating a new content item) can be achieved using the `Try it out` button. This is absolutely the case here. I really think there is only a handful of endpoints for which we should be using a body param. A good example of an endpoint that should be using a body param is `/docs/rest/#!/content/postContentContentIdMembers_post_15`. Because of the dynamic parameter, the action can not be accomplished at all using form params.