add docstring

Author: cecille

src/python_testing/taglist_and_topology_test_support.py

@@ -192,6 +192,7 @@ def find_tag_list_problems(roots: list[int], device_types: dict[int, dict[int, s
 
 
 def flat_list_ok(flat_endpoint_id_to_check: int, endpoints_dict: dict[int, Any]) -> bool:
+    '''Checks if the (flat) PartsList on the supplied endpoint contains all the sub-children of its parts.'''
     sub_children = set()
     for child in endpoints_dict[flat_endpoint_id_to_check][Clusters.Descriptor][Clusters.Descriptor.Attributes.PartsList]:
         sub_children.update(get_all_children(child, endpoints_dict))