[1.10.x] Fixed #27294 -- Documented UserCreationForm's fields.

Backport of 1d25eb9688 from master

docs/topics/auth/default.txt

@@ -1593,7 +1593,14 @@ provides several built-in forms located in :mod:`django.contrib.auth.forms`:
 
 .. class:: UserCreationForm
 
-    A form for creating a new user.
+    A :class:`~django.forms.ModelForm` for creating a new user.
+
+    It has three fields: ``username`` (from the user model), ``password1``,
+    and ``password2``. It verifies that ``password1`` and ``password2`` match,
+    validates the password using
+    :func:`~django.contrib.auth.password_validation.validate_password`, and
+    sets the user's password using
+    :meth:`~django.contrib.auth.models.User.set_password()`.
 
 .. currentmodule:: django.contrib.auth