Better document API expectations

It came up recently that we don't document API expectations (e.g https://github.com/astropy/astroquery/issues/1523), e.g returning `NoResultsWarning`s for empty results rather than errors, etc.

Start by reviewing issue 1523 and the existing astroquery API documentation to identify where expectations are currently missing. Define the documentation scope around behaviors such as returning NoResultsWarnings for empty results rather than errors; done means the relevant API expectations are consistently documented.

Written by the indexing model from the issue text.