DOCS: constants are not documented in API/narrative docs

e.g. `POLARIZATION_STATES` or `CALIBRATION_LEVELS` are not documented, yet referred to in API docs:

https://pyvo.readthedocs.io/en/latest/api/pyvo.dal.imagesearch2.html#pyvo.dal.imagesearch2

It would be nice to have those link work to docs them.

Start with the pyvo.dal.imagesearch2 API documentation page linked in the issue and trace the references to POLARIZATION_STATES and CALIBRATION_LEVELS. Done means each referenced constant has a working documentation link and its documentation is discoverable from the API narrative.

Written by the indexing model from the issue text.