Update arango/collection.py

Co-authored-by: Anthony Mahanna <43019056+aMahanna@users.noreply.github.com>

Author: apetenchea

arango/collection.py

@@ -574,6 +574,7 @@ def truncate(
         """Delete all documents in the collection.
 
         :param sync: Block until deletion operation is synchronized to disk.
+        :type sync: bool | None
         :param compact: Whether to compact the collection after truncation.
         :return: True if collection was truncated successfully.
         :rtype: bool