Refs #26167 -- Functional Indexes don't have a fields attribute

Referring to that attribute in AddIndex.describe() will thus not work.
The description of an index should instead live in the index class
definition of the index.

django/db/migrations/operations/models.py

@@ -784,9 +784,8 @@ def deconstruct(self):
         )
 
     def describe(self):
-        return 'Create index %s on field(s) %s of model %s' % (
+        return 'Create index %s for model %s' % (
             self.index.name,
-            ', '.join(self.index.fields),
             self.model_name,
         )