pgsql: doc: Add note about pg_settings and customized options into cata

doc: Add note about pg_settings and customized options into catalogs.sgml.

The pg_settings view does not display customized options until
the extension module that defines them has been loaded. This commit
add the note about that behavior, into the docs.

Author: John Naylor
Reviewed-by: Tom Lane, Fujii Masao
Discussion: https://postgr.es/m/CAFBsxsGsBZsG=cLM0Op5HFb2Ks6SzJrOc_eRO_jcKSNuqFRKnQ@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ef60de67ebde6dfd1ea09b4d08166736bf05698c

Modified Files
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doc/src/sgml/catalogs.sgml | 12 ++++++++----
1 file changed, 8 insertions(+), 4 deletions(-)