Doc: Improve description of the "batch_size" option for postgres_fdw.
Document that the actual number of rows postgres_fdw inserts at once in
the COPY case is determined in a similar way to the INSERT case, but it
has a restriction that does not apply to the INSERT case.
Follow-up for commit
97da48246.
Reviewed-by: Daniel Gustafsson and Tatsuo Ishii
Discussion: https://postgr.es/m/CAPmGK14NMXDMW4qK9kHUzudN9t71uvrMKPna02X6zwgQJ6E1_g%40mail.gmail.com