Where can I find a comprehensive description of the search filters for search REST API?

I'm a developer at Social Edge Consulting and I'm going to build new widget, the advanced search widget. For that I want to use REST API with calls being sent from frontend JS. I need to set many different filters for the search, for example I need to know how (if?) I can get only the answered questions in search results.

Even if platform rules suggest to use Widget API instead, search API there also doesn't seem to have any "Filter" field description.