I'd like to find this function soptoolutils.computeRange() and see the code. Is there any kind of documentation?

found this : https://jtomori.github.io/houdini_additional_python_docs/soptoolutils.html [jtomori.github.io]

but the function is not included

There is no official documentation for soptoolutils and other additional python packages. They evolve with newer Houdini versions, so it makes sense that you don't find the function you are looking for in this link, probably built with older Houdini version.

You can find soptoolutils and other python packages under "$HFS/houdini/pythonX.Ylibs", for example with Windows and Houdini 20.0.590 it's placed under "C:/Program Files/Side Effects Software/Houdini 20.0.590/houdini/python3.10libs"

If you use an IDE to read python files in this folder you could see how functions are written.