doc: Document DEFAULT option in file_fdw.

doc: Document DEFAULT option in file_fdw.

Commit 9f8377f7a introduced the DEFAULT option for file_fdw but did not
update the documentation. This commit adds the missing description of
the DEFAULT option to the file_fdw documentation.

Backpatch to v16, where the DEFAULT option was introduced.

Author: Shinya Kato <shinya11.kato@gmail.com>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/CAOzEurT_PE7QEh5xAdb7Cja84Rur5qPv2Fzt3Tuqi=NU0WJsbg@mail.gmail.com
Backpatch-through: 16