Include filter/field options in the documentation about unipressed

[multimeric]

This is actually quite a difficult problem. I can transform the auto-generated mkdocstrings docs, but it's not clear if I should:

• List out all types, which would be comprehensive but unreadable
• Simplify the top-level literals such as the UniprotKB fields into a single list of options. This is possible, I wrote a transformer that does this, but it's not clear how to present this information. The rest of the docs present data structures in terms of Python code, but "a list of strings" isn't an actual data type we ever work with, in reality it's a type literal
• Generate docs for the .search method of each dataset client separately, and add the list of options to the docstring. This might be user friendly, but the number of options (sometimes in the hundreds) might get unwieldy.