doc: add PARALLEL option to old syntax for CREATE AGGREGATE

The old pre-8.2 syntax for CREATE AGGREGATE also support PARALLEL,
even though it's required to be quoted as PARALLEL is an unreserved
keyword.

Author: danielgustafsson

doc/src/sgml/ref/create_aggregate.sgml

@@ -80,6 +80,7 @@ CREATE [ OR REPLACE ] AGGREGATE <replaceable class="parameter">name</replaceable
     [ , MFINALFUNC_MODIFY = { READ_ONLY | SHAREABLE | READ_WRITE } ]
     [ , MINITCOND = <replaceable class="parameter">minitial_condition</replaceable> ]
     [ , SORTOP = <replaceable class="parameter">sort_operator</replaceable> ]
+    [ , "PARALLEL" = { SAFE | RESTRICTED | UNSAFE } ]
 )
 </synopsis>
  </refsynopsisdiv>
@@ -641,6 +642,10 @@ SELECT col FROM tab ORDER BY col USING sortop LIMIT 1;
       functions are not consulted by the planner, only the marking of the
       aggregate itself.
      </para>
+     <para>
+       Note that when using the old syntax, <literal>"PARALLEL"</literal> must
+       be quoted.
+     </para>
     </listitem>
    </varlistentry>