I'm looking for the reference/documentation of the dos.h
API.
(especially for an openwatcom specific version).
Where to get the documentation?
Using the int 21h function reference widely available you could write your own DOS interface routine where the parameters are loaded into the appropriate registers inside the routine.
As I recall I needed three functions: one for character IO (byte register parameters), one for semi-complex (word register parameters - files, memory) and a third which I can no longer remember :(
It's a good exercise!
http://poli.cs.vsb.cz/c/help/dos.htm
Almost certainly not the original but a good resource anyway
One reasonable starting point would probably be the library documentation at OpenWatcom.