Previously, the setup was confusing. Every symbol imported to module
level was documented directly. If a ToC was added, sphinx would fail
due to documentation generated twice for the same symbol.

The new setup **requires** a dedicated autosummary selection of
documented content.

I think this is better. Often modules provide a lot of stuff, and
direct documentation rendering on a single page is overwhelming.
The explicit selection also allows for items to be excluded
that are only there for technical or historical reasons. This allows for
an overall more accessible documentation.

We have no other choice, but to modify them. Even for the legacy
adaptor, we do not want to maintain senseless complexities.
The changes in the tests will document the exact differences.

This will eventually fix
datalad#397