Issue #23662: Document default value of RFC 2109 attributes

Doc/library/http.cookies.rst

@@ -141,7 +141,7 @@ Morsel Objects
    in HTTP requests, and is not accessible through JavaScript. This is intended
    to mitigate some forms of cross-site scripting.
 
-   The keys are case-insensitive.
+   The keys are case-insensitive and their default value is ``''``.
 
    .. versionchanged:: 3.5
       :meth:`~Morsel.__eq__` now takes :attr:`~Morsel.key` and :attr:`~Morsel.value`