gh-89735: Document requirements for the *headers* argument (GH-155640)

Both handlers look up the header name in lowercase, and the basic
one needs get_all(), so a plain dict does not work.

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: serhiy-storchaka

Doc/library/urllib.request.rst

@@ -1056,6 +1056,10 @@ AbstractBasicAuthHandler Objects
    authenticate for, *req* should be the (failed) :class:`Request` object, and
    *headers* should be the error headers.
 
+   *headers* must be a mapping-like object with case-insensitive lookup
+   that implements the ``get_all()`` method,
+   such as :class:`email.message.Message` or :class:`wsgiref.headers.Headers`.
+
    *host* is either an authority (e.g. ``"python.org"``) or a URL containing an
    authority component (e.g. ``"https://python.org/"``). In either case, the
    authority must not contain a userinfo component (so, ``"python.org"`` and
@@ -1097,6 +1101,9 @@ AbstractDigestAuthHandler Objects
    should be the (failed) :class:`Request` object, and *headers* should be the
    error headers.
 
+   *headers* must be a mapping-like object with case-insensitive lookup,
+   such as :class:`email.message.Message` or :class:`wsgiref.headers.Headers`.
+
 
 .. _http-digest-auth-handler: