[ADD] document @default_order in list views

closes #2623

doc/reference/views.rst

@@ -132,6 +132,14 @@ root can have the following attributes:
     <reference/views/form>`'s fields and buttons are thus accepted by list
     views although they may not have any meaning if the list view is
     non-editable
+``default_order``
+    overrides the ordering of the view, replacing the model's default order.
+    The value is a comma-separated list of fields, postfixed by ``desc`` to
+    sort in reverse order:
+
+    .. code-block:: xml
+
+        <tree default_order="sequence,name desc">
 ``colors``
     allows changing the color of a row's text based on the corresponding
     record's attributes.