JupyterNotebooksLib is a tiny set of convenience classes. We haven’t set up a readthedocs site for it,. You can type the classname and press Shift-Tab for documentation:
You can also browse the documented source code here: SlicerJupyter/JupyterNotebooks/JupyterNotebooksLib/widgets.py at master · Slicer/SlicerJupyter · GitHub
If anything is not clear or incomplete then feel free to ask here. If you figure out something or your questions are answered then it would be great if you could send pull requests with your suggested changes to the documentation.