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