Yes AFAIK there is no .usc manual. All I have learned has been by reading existing files and trying.
You can put directly a class derived from Ctrl without .usc in the Layout Editor: just choose the first option "User Class" and enter your class name in class name field.
About SliderCtrlX, although the code is not mine, I put it in Bazaar trying unsuccessfully to force to put the improvements in the official source. I will put now a basic .usc just to help.