Change the documentation for how ext_pillar is called

Make it clear that the first argument passed is ``pillar``. Also
explain why it makes more sense to return a dictionary of just
the pillar values a particular ext_pillar module is changing or
adding, rather than returning the entire ``pillar`` dictionary.

Note: it isn't documented anywhere that you can use earlier pillar
dictionary values in an external pillar, although that is what the
code does.