[1.5.X] Fixed #19161 - Added missing clean_password method in custom user docs

Thanks DavidW for the report.

Backport of e8f696097b from master

docs/topics/auth.txt

@@ -408,7 +408,7 @@ installation supports. The first entry in this list (that is,
 ``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::
 
@@ -2283,13 +2283,19 @@ code would be required in the app's ``admin.py`` file::
     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