add document show function for doc_type, date and domain

Useful if all you want is a doc's domain / doc_type / date. See [pillowtop retry](https://github.com/dimagi/pillowtop/pull/89/files#diff-6dac4be25b780fb79c406285662331fcR300) for an example use.

REQUIRES REINDEX
@dannyroberts yes, I noticed those as well. The 'recommended' approach
there would be to create a 'list function' which can render the results of
a view. I'm not sure what the performance of list functions is though but
will put it on my list of things to test out.