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

doc/src/sgml/postgres-fdw.sgml

@@ -437,7 +437,11 @@ OPTIONS (ADD password_required 'false');
       </para>
 
       <para>
-       This option also applies when copying into foreign tables.
+       This option also applies when copying into foreign tables.  In that case
+       the actual number of rows <filename>postgres_fdw</filename> copies at
+       once is determined in a similar way to the insert case, but it is
+       limited to at most 1000 due to implementation restrictions of the
+       <command>COPY</command> command.
       </para>
      </listitem>
     </varlistentry>