Doc: Update documentation for modifying postgres_fdw foreign tables.

Document that they can be modified using COPY as well.

Back-patch to v11 where commit 3d956d95 added support for COPY in
postgres_fdw.

doc/src/sgml/postgres-fdw.sgml

@@ -64,7 +64,8 @@
   Now you need only <command>SELECT</command> from a foreign table to access
   the data stored in its underlying remote table.  You can also modify
   the remote table using <command>INSERT</command>, <command>UPDATE</command>,
-  <command>DELETE</command>, or <command>TRUNCATE</command>.
+  <command>DELETE</command>, <command>COPY</command>, or
+  <command>TRUNCATE</command>.
   (Of course, the remote user you have specified in your user mapping must
   have privileges to do these things.)
  </para>