Clarify constraints on update/delete

The update/delete actions could use some clarifications describing required constraints. Ideas are described here: https://developer.jboss.org/thread/250173

The atom spec's "2.3.5.5 Deleting an Artifact Entry" mentions "If an artifact is deleted which is the Target Entry of a relationship instance owned by some other Source Entry, then that relationship instance is also deleted in that Source Entry.". That needs clarified, based on possible constraints.