Doc: Further update documentation for asynchronous execution.
Add a note about asynchronous execution by postgres_fdw when applied to
Append nodes that contain synchronous subplan(s) as well.  Follow-up for
commit
Andrey Lepikhov and Etsuro Fujita
Discussion:
@@ -411,6 +411,19 @@ OPTIONS (ADD password_required 'false');
In such a case, it may be more performant to disable this option to
eliminate the overhead associated with running queries asynchronously.
</para>
+
+ <para>
+ Asynchronous execution is applied even when an
+ <structname>Append</structname> node contains subplan(s) executed
+ synchronously as well as subplan(s) executed asynchronously.
+ In such a case, if the asynchronous subplans are ones processed using
+ <filename>postgres_fdw</filename>, tuples from the asynchronous
+ subplans are not returned until after at least one synchronous subplan
+ returns all tuples, as that subplan is executed while the asynchronous
+ subplans are waiting for the results of asynchronous queries sent to
+ foreign servers.
+ This behavior might change in a future release.
+ </para>
</listitem>
</varlistentry>
