Fix pg_description entries for jsonb_to_record() and jsonb_to_recordset().

Fix pg_description entries for jsonb_to_record() and jsonb_to_recordset().

All the other jsonb function descriptions refer to the arguments as being
"jsonb", but these two said "json".  Make it consistent.  Per bug #13905
from Petru Florin Mihancea.

No catversion bump --- we can't force one in the back branches, and this
isn't very critical anyway.