info on unit WIN31.PAS

I found a routine in the unit WIN31.PAS in directory \tpw\win in
TPW 1.5 which appears to have a function that does just what I want.
But I can't find any documentation on this unit or the functions it
exports.  (The function I need is GetSelectorBase.)

Are these routines documented anywhere?  I know these are really
interfaces to DPMI calls provided by windows, where are they documented?

Thanks in advance.                      Jon