pgsql: 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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ad381d0d9244b1490efccca2126ba3f82e144605

Modified Files
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doc/src/sgml/file-fdw.sgml | 11 +++++++++++
1 file changed, 11 insertions(+)