Uh, in my source tree that is a symlink to src/backend/catalog/pg_type_d.h, but that file is generated by src/backend/catalog/genbki.pl. Does that help you?
Shouldn't we replace "are defined in the file" with "are defined in the generated file"? That's the only place that mentions *_d.h files. Perhaps adding a link to BKI [1] should avoid further questions.