[1.6.x] Fixed #20762 -- Clarified that add_fieldsets is not a standard ModelAdmin attribute.

Backport of dce8945ea6 from master

docs/topics/auth/customizing.txt

@@ -1094,6 +1094,8 @@ code would be required in the app's ``admin.py`` file::
             ('Personal info', {'fields': ('date_of_birth',)}),
             ('Permissions', {'fields': ('is_admin',)}),
         )
+        # add_fieldsets is not a standard ModelAdmin attribute. UserAdmin
+        # overrides get_fieldsets to use this attribute when creating a user.
         add_fieldsets = (
             (None, {
                 'classes': ('wide',),