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 27e1f145.

Andrey Lepikhov and Etsuro Fujita

Discussion: https://postgr.es/m/58fa2aa5-07f5-80b5-59a1-fec8a349fee7%40postgrespro.ru