PQftype, Oid, where is this documented?

Hi --

I'm using both the perl Pg interface and the C libpq-fe interface to
PostGreSQL (version 6.5).  Both of these have a "ftype" function which
retunrs the type of a field.  They are documented as returning an "Oid".
However, I haven't found where the Oid values themselves are docuemnted.

I could "reverse engineer" the thing by looking at what I get for various
data types and using that, but I'd feel more comfortable using documented
values.  Is there documentation anywhere that tells you how to interpret
the Oid you get back from PQftype()?

Thanks,

-Rob