pgsql: docs: Improve pg_settings_get_flags docs.

docs: Improve pg_settings_get_flags docs.

In the docs, the GUC flags that pg_settings_get_flags() reported were
listed using <simplelist>. But the list was treated as separate lines
in the existing function table and didn't look good. For better view,
this commit separates the list from the table entry for
pg_settings_get_flags() and adds the table for it at the bottom of
the existing function table.

Author: Fujii Masao
Reviewed-by: Alvaro Herrera, Michael Paquier
Discussion: https://postgr.es/m/f093edf9-6e5a-b119-ee50-6a2c97c79ee8@oss.nttdata.com

Back-patch of f2d0c7f18 into v15.
Discussion: https://postgr.es/m/20221103123320.GQ16921@telsasoft.com

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/387e059f8e9c506aba17d63e7220cad2f488f238

Modified Files
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doc/src/sgml/func.sgml | 64 +++++++++++++++++++++++++++++++++-----------------
1 file changed, 42 insertions(+), 22 deletions(-)