pgsql: Doc: Improve description of the "batch_size" option for postgres

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/0f0a7183d7d8632a2bbcdad796580b4cc541c639

Modified Files
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doc/src/sgml/postgres-fdw.sgml | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)