`REF` edge from `CALL` to members is not documented

[fabsx00]

There are currently outgoing REF edges from CALL to members in case of member accesses, but information about this is not in the documentation. This also seems a bit non-standard, as it only exists for member accesses. We should either document this, or take this as an opportunity to simplify: are these REF edges really required, and if so, would it possibly make more sense to create them from the call arguments to members?