[2.1.x] Refs #26608 -- Removed incorrect sentence in Expression.contains_over_clause docs.

Backport of ab251fdad251cfb1e9fb61c42b5bfed9d0afe393 from master

docs/ref/models/expressions.txt

@@ -907,7 +907,7 @@ calling the appropriate methods on the wrapped expression.
         Tells Django that this expression contains a
         :class:`~django.db.models.expressions.Window` expression. It's used,
         for example, to disallow window function expressions in queries that
-        modify data. Defaults to ``True``.
+        modify data.
 
     .. attribute:: filterable