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

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

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/de2d1920ddf54ec8dbcc87821ade5346f0cf8926

Modified Files
--------------
doc/src/sgml/config.sgml   | 2 ++
doc/src/sgml/file-fdw.sgml | 4 +++-
2 files changed, 5 insertions(+), 1 deletion(-)