I had to look when you asked - but it is on the Sinclair QL Homepage:
http://www.dilwyn.me.uk/database/index.html
Look for DdesApi.zip under DataDesign.
This includes a PDF of teh API programming manual - the description of the commands starts on page 16. Each command has the information for use from SuperBASIC and from Assembler.
Was it something else you are looking for?