Mention Ellipsis pickling in the docs (pythonGH-103660)

(cherry picked from commit 14ec0bb)

Co-authored-by: sterliakov <50529348+sterliakov@users.noreply.github.com>

Doc/library/pickle.rst

@@ -494,7 +494,8 @@ What can be pickled and unpickled?
 
 The following types can be pickled:
 
-* ``None``, ``True``, and ``False``;
+* built-in constants (``None``, ``True``, ``False``, ``Ellipsis``, and
+  ``NotImplemented``);
 
 * integers, floating-point numbers, complex numbers;