Update gencost docs warning

docs/source/mods/opf/opf.rst

@@ -164,12 +164,11 @@ an example case with 9 buses and displays the key fields:
 
         .. warning::
 
-            The Mod only supports generator costs with ``costtype = 2`` i.e.,
-            polynomial model, up to degree 2, i.e, ``n=3`` in the ``gencost``
-            structure. For now, these seem to be the most commonly used settings
-            in practice. If a different costtype or ``n`` value is provided, an
-            error is issued. It is possible that more functionality will be
-            added in a future release.
+            The Mod only supports generator costs with ``costtype = 2`` and ``n
+            <= 3`` (i.e. linear and quadratic cost functions) in the ``gencost``
+            structure, as these are the most commonly used settings in practice.
+            If a different costtype or larger value of ``n`` value is provided,
+            an error will be issued.
 
 
 Cases can also be read directly from MATPOWER format using