Traversing the catalog using heap_open, systable_beginscan, ...

I am writing a foreign data wrapper and following the oracle example. Are there docs that discuss heap_open, systable_beginscan and systable_getnext?

I currently just need to do simple stuff, like translate an Oid that is received via a FDW callback into the text name of a table. I see all of the catalog headers, pg_class.h, etc... but not seeing the pattern of which args to pass into ScanKeyInit to traverse the catalog correctly.

Hints greatly appreciated.

If there is a better list, please point me to it.

Thanks,

Mark