pgsql: Doc: Further update documentation for asynchronous execution.

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 27e1f1456.

Andrey Lepikhov and Etsuro Fujita

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/eab81953682d5087295afb911c93f36cb1533bd9

Modified Files
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doc/src/sgml/postgres-fdw.sgml | 13 +++++++++++++
1 file changed, 13 insertions(+)