Document how to set-up permissions for event callback API

The existing version of pub/sub permissions documentation seems is not complete enough. In scope of MODPUBSUB-93 we realized that a permission for the API, that will be used as callback, has to be defined in mod-pubsub descriptor (vs in its own descriptor as usual). This note has to be added to the doc. (what doc?)

NOTE: Review with Vega during knowledge transfer; perhaps the teams can together start a draft wiki page with documentation and continue to add to it over the course of a few sprints during Juniper/Kiwi transition time.