doc: cross-link file-fdw and CSV config log sections

There is an file-fdw example that reads the server config file, so cross
link them.

Reported-by: Oleg Samoilov

Discussion: https://postgr.es/m/159800192078.2886.10431506404995508950@wrigleys.postgresql.org

Backpatch-through: 9.5

doc/src/sgml/config.sgml

@@ -6689,6 +6689,8 @@ CREATE TABLE postgres_log
 <programlisting>
 COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
 </programlisting>
+        It is also possible to access the file as a foreign table, using
+        the supplied <xref linkend="file-fdw"/> module.
        </para>
 
        <para>

doc/src/sgml/file-fdw.sgml

@@ -216,7 +216,9 @@
   <para>
    One of the obvious uses for <literal>file_fdw</literal> is to make
    the PostgreSQL activity log available as a table for querying.  To
-   do this, first you must be logging to a CSV file, which here we
+   do this, first you must be <link
+   linkend="runtime-config-logging-csvlog">logging to a CSV file,</link>
+   which here we
    will call <literal>pglog.csv</literal>.  First, install <literal>file_fdw</literal>
    as an extension:
   </para>