Doc: clarify use of RECURSIVE in WITH.

Apparently some people misinterpreted the syntax as being that
RECURSIVE is a prefix of individual WITH queries.  It's a modifier
for the WITH clause as a whole, so state that more clearly.

Discussion: https://postgr.es/m/ca53c6ce-a0c6-b14a-a8e3-162f0b2cc119@a-kretschmer.de

doc/src/sgml/ref/select.sgml

@@ -281,6 +281,14 @@ TABLE [ ONLY ] <replaceable class="parameter">table_name</replaceable> [ * ]
     reads all or any of their output.
    </para>
 
+   <para>
+    When there are multiple queries in the <literal>WITH</literal>
+    clause, <literal>RECURSIVE</literal> should be written only once,
+    immediately after <literal>WITH</literal>.  It applies to all queries
+    in the <literal>WITH</literal> clause, though it has no effect on
+    queries that do not use recursion or forward references.
+   </para>
+
    <para>
     The primary query and the <literal>WITH</literal> queries are all
     (notionally) executed at the same time.  This implies that the effects of