Add missing pg_description strings for pg_type entries.

Add missing pg_description strings for pg_type entries.

I noticed that all non-composite, non-array entries in pg_type.dat
had descr strings, except for "json" and the pseudo-types.  The
lack for json seems certainly an oversight, and there's surely
little reason to not have entries for the pseudo-types either.
So add some.

"make reformat-dat-files" turned up some formatting issues in
pg_amop.dat, too, so fix those in passing.

No catversion bump since the backend doesn't care too much what is
in pg_description.