Fixed #19161 - Added missing clean_password method in custom user docs

``settings.PASSWORD_HASHERS[0]``) will be used to store passwords, and all the

other entries are valid hashers that can be used to check existing passwords.

This means that if you want to use a different algorithm, you'll need to modify

:setting:`PASSWORD_HASHERS` to list your prefered algorithm first in the list.

:setting:`PASSWORD_HASHERS` to list your preferred algorithm first in the list.

The default for :setting:`PASSWORD_HASHERS` is::

    class UserChangeForm(forms.ModelForm):

        """A form for updateing users. Includes all the fields on

        the user, but replaces the password field with admin's

        pasword hash display field.

        password hash display field.

        """

        password = ReadOnlyPasswordHashField()

        class Meta:

            model = MyUser

        def clean_password(self):

            # Regardless of what the user provides, return the initial value.

            # This is done here, rather than on the field, because the

            # field does not have access to the initial value

            return self.initial["password"]

    class MyUserAdmin(UserAdmin):

        # The forms to add and change user instances