Document use of GEQO param in subquery pullups.

doc/src/sgml/runtime.sgml

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 <Chapter Id="runtime">
@@ -719,7 +719,9 @@ env PGOPTIONS='-c geqo=off' psql
         this many FROM items involved.  (Note that a JOIN construct
 	counts as only one FROM item.) The default is 11. For simpler
 	queries it is usually best to use the
-        deterministic, exhaustive planner.
+        deterministic, exhaustive planner.  This parameter also controls
+        how hard the optimizer will try to merge subquery
+        <literal>FROM</literal> clauses into the upper query.
        </para>
       </listitem>
      </varlistentry>