How to document permission types

Post a discussion with @micbar I made some thoughts how permission types defined in the [settings service](https://github.com/owncloud/ocis/blob/39f227701bc6b89c7b4315aed9473d4bd54e46d6/services/settings/pkg/store/defaults/defaults.go) could be documented. After a short sync with @kobergj, we came to following conclusion:

* To be future proof, we should have an automated process that generates an adoc table based on file content like we do with other data which than can be included by the admin docs.
* In the admin docs, we generate a page/section (tbd) with some description and include that table.
* We need a one-time fix in defaults.go as stuff is not consitently implemented like speaking names or missing descriptions.

Having that, future changes get auto-documented without additional effort.

This task can be started post releasing 3.0 but should be considered to be implemented.

@micbar @dragotin
@kobergj @dragonchaser FYI