[doc] add description of method EngineProcessor.get_params()

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

searx/search/processors/online_url_search.py

@@ -20,6 +20,9 @@ class OnlineUrlSearchProcessor(OnlineProcessor):
     engine_type = 'online_url_search'
 
     def get_params(self, search_query, engine_category):
+        """Returns a set of *request params* or ``None`` if search query does not match
+        to at least one of :py:obj:`re_search_urls`.
+        """
         params = super().get_params(search_query, engine_category)
         if params is None:
             return None