pgsql: Doc: fix mistaken reference to "PG_ARGNULL_xxx()" macro.

Doc: fix mistaken reference to "PG_ARGNULL_xxx()" macro.

This should of course be just "PG_ARGISNULL()".

Also reorder a couple of paras to make the discussion of PG_ARGISNULL
less disjointed.

Back-patch to v10 where the error was introduced.

Laurenz Albe and Tom Lane, per an anonymous docs comment

Discussion: https://postgr.es/m/158399487096.5708.10696365251766477013@wrigleys.postgresql.org

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/7e32aaab589a8927c22d3b83d55fc0e77424b348

Modified Files
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doc/src/sgml/xfunc.sgml | 20 ++++++++++----------
1 file changed, 10 insertions(+), 10 deletions(-)