Read a parameter's NULL note through the markup around it

[estebanzimanyi]

Doxygen marks up the literal a description names, so one note is written four
ways — NULL, NULL, @p NULL and \p NULL — and the extractor reads only
the bare and backticked pair. Eighteen parameters over sixteen functions carry
a note an author marked up, and every one of them reaches the bindings as a
parameter that must be supplied: the spheroid interpolate_point4d_spheroid
takes the sphere path without, the measure of the nine *_to_geomeas walks,
the cluster flag of geo_union_dbscan, and the identifier and distance a
nearest-neighbour cursor step writes.

shape.nullable carries 131 parameters over 102 functions, against 113 over 86,
and no parameter loses the note it had. The suite states the markup forms beside
the plain one, on a function whose third parameter carries no note at all.

[estebanzimanyi]

Nullability derives from the SQL layer. PostgreSQL enforces STRICT at run time, so the declaration states the contract that a prose note only describes: 8201 of 8587 CREATE FUNCTION statements carry STRICT, 386 do not, and 154 PG_ARGISNULL guards name the arguments those admit, alongside 1136 parameters carrying a SQL DEFAULT. Reading five prose spellings of one note keeps prose as the source; what a generator reads is the SQL declaration and a tag beside the aliases doxygen/Doxyfile already defines.