Add support and documentation for export from HDMF version 2

[rly]

Motivation

See hdmf-dev/hdmf#388

I also changed the docval for NWBHDF5IO.export from what it was for HDF5IO.export:

HDF5IO.export(path=..., container=...)
NWBHDF5IO.export(path=..., nwbfile=...)

Checklist

• Have you checked our Contributing document?
• Have you ensured the PR description clearly describes problem and the solution?
• Is your contribution compliant with our coding style? This can be checked running flake8 from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• Have you included the relevant issue number using #XXX notation where XXX is the issue number?

[oruebel]

Aside from the minor comment the tutorial description this looks good.

[rly]

@oruebel I made the suggested changes and added another section on Object IDs