Doc: Improve note about copying into postgres_fdw foreign tables in batch

Hi,

Here is a small patch to improve the note, which was added by commit
97da48246 ("Allow batch insertion during COPY into a foreign table."),
by adding an explanation about how the actual number of rows
postgres_fdw inserts at once is determined in the COPY case, including
a limitation that does not apply to the INSERT case.

I will add this to the next CF.

Best regards,
Etsuro Fujita