pgsql: doc: Add example for how to set file_fdw column option

doc: Add example for how to set file_fdw column option

The documentation is pretty light on how to set column options
on foreign tables, and the file_fdw docs refer to COPY when
documenting force_null even though it's not used in the same
way. Add a small example to describe how to use it.

Reported-by: Boshomi Phenix <boshomi@gmail.com>
Discussion: https://postgr.es/m/CAJVkCUparn4_Oarernm=U6LWVsTkecKcALHtwGr5M3qJRj_czw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/51908a37d7ea67655560ff2efcba66816ca458e2

Modified Files
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doc/src/sgml/file-fdw.sgml | 16 ++++++++++++++++
1 file changed, 16 insertions(+)