add comment about backwards compatibility

Author: mishaschwartz

pystac_client/client.py

@@ -457,6 +457,8 @@ def get_items(self, *ids: str, recursive: bool = True) -> Iterator["Item_Type"]:
                 catalog.
         """
         if self.conforms_to(ConformanceClasses.ITEM_SEARCH):
+            # Previously, recursive=None was treated the same as recursive=True.
+            # This if statement maintains this behaviour for backwards compatibility.
             if recursive is not False:
                 search = self.search(ids=ids)
                 try: