DAS-2065: Adds types

Author: flamingbear

xarray/core/datatree_ops.py

@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 import re
 import textwrap
 
@@ -215,7 +217,7 @@ def method_name(self, *args, **kwargs):
                 setattr(target_cls_dict[method_name], "__doc__", new_method_docstring)
 
 
-def insert_doc_addendum(docstring, addendum):
+def insert_doc_addendum(docstring: str | None, addendum: str) -> str | None:
     """Insert addendum after first paragraph or at the end of the docstring.
 
     There are a number of Dataset's functions that are wrapped. These come from