[CF 5650] v1 - Parallel safety docs for CTEs

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Patch(es): https://www.postgresql.org/message-id/CAAaqYe_TjZugK5JmiBfNHBKEDi5N+AeQ5UNGisWcqe7wvtQy4Q@mail.gmail.com
Author(s): James Coleman

Author: Commitfest Bot

doc/src/sgml/parallel.sgml

@@ -493,7 +493,7 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
   <itemizedlist>
     <listitem>
       <para>
-        Scans of common table expressions (CTEs).
+        Scans of common table expressions (CTEs) which must be materialized.
       </para>
     </listitem>