Add documentation about with clause non-determinism
The current behavior of the with clause, where the query gets executed
each time the relation is used is contrary to the SQL standard.  Add
warning about it to the docs.
@@ -84,6 +84,11 @@ Additionally, the relations within a ``WITH`` clause can chain::
z AS (SELECT b AS c FROM y)
SELECT c FROM z;
+.. WARNING::
+ Currently, the SQL for the ``WITH`` clause will be inlined anywhere the named
+ relation is used. This means that if the relation is used more than once and the query
+ is non-deterministic, the results may be different each time.
+
GROUP BY Clause
---------------
