[cleanup] Add keyword automatically to SearchIE descriptions

and some minor cleanup of docs

yt_dlp/extractor/common.py

@@ -3620,9 +3620,11 @@ class SearchInfoExtractor(InfoExtractor):
     """
     Base class for paged search queries extractors.
     They accept URLs in the format _SEARCH_KEY(|all|[0-9]):{query}
-    Instances should define _SEARCH_KEY and _MAX_RESULTS.
+    Instances should define _SEARCH_KEY and optionally _MAX_RESULTS
     """
 
+    _MAX_RESULTS = float('inf')
+
     @classmethod
     def _make_valid_url(cls):
         return r'%s(?P<prefix>|[1-9][0-9]*|all):(?P<query>[\s\S]+)' % cls._SEARCH_KEY