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