Update docstring for DiffElement

Author: dgarros

dsync/diff.py

@@ -143,6 +143,7 @@ def __init__(
             keys: Primary keys and values uniquely describing this object, as in DSyncModel.get_identifiers().
             source_name: Name of the source DSync object
             dest_name: Name of the destination DSync object
+            diff_class: Diff or subclass thereof to use to calculate the diffs to use for synchronization 
         """
         if not isinstance(obj_type, str):
             raise ValueError(f"obj_type must be a string (not {type(obj_type)})")