[5.1.x] Clarified docs for default email value in UserManager.create_user().

Backport of 5da3ad7 from main.

Author: amirreza8002

docs/ref/contrib/auth.txt

@@ -284,6 +284,9 @@ Manager methods
         :meth:`~django.contrib.auth.models.User.set_unusable_password()` will
         be called.
 
+        If no email is provided, :attr:`~django.contrib.auth.models.User.email`
+        will be set to an empty string.
+
         The ``extra_fields`` keyword arguments are passed through to the
         :class:`~django.contrib.auth.models.User`’s ``__init__`` method to
         allow setting arbitrary fields on a :ref:`custom user model