Doc: Improve description of the "batch_size" option for postgres_fdw.

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