pgsql: Doc: improve xfunc-c-type-table.

Doc: improve xfunc-c-type-table.

List types numeric and timestamptz, which don't seem to have ever been
included here.  Restore bigint, which was no-doubt-accidentally deleted
in v12.  Fix some errors, or at least obsolete usages (nobody declares
float arguments as "float8*" anymore, even though they might be that
under the hood).  Re-alphabetize.  Remove the seeming claim that this
is a complete list of built-in types.

Per question from Oskar Stenberg.

Discussion: https://postgr.es/m/HE1PR03MB2971DE2527ECE1E99D6C19A8F96E9@HE1PR03MB2971.eurprd03.prod.outlook.com

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/8a291f4e50b3ff2f2748998a50dd4bcef5623384

Modified Files
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doc/src/sgml/xfunc.sgml | 53 ++++++++++++++++++++++++++++---------------------
1 file changed, 30 insertions(+), 23 deletions(-)