For the Pico I had to dig into the source-code: pimoroni-pico/drivers/ioexpander/ioexpander.cpp at dc93a19848e7a3865625effb46d248589d2ae214 · pimoroni/pimoroni-pico · GitHub
The normal python-library has a library-reference, which is here: ioe-python/REFERENCE.md at main · pimoroni/ioe-python · GitHub
You have to scroll down to the bottom to get a list of methods. Regarding documentation, this is extremely minimalistic though. And I did not find an example.
To be honest, I did not search for that method, I stumbled over it and that is why I posted it - knowing how hard it is to find.