OpenCV's documentation can be found at http://docs.opencv.org/, where you can either read it online or download it for offline reading. If you write code on airplanes or other places without internet access, you will definitely want to keep offline copies of the documentation.
The documentation includes a combined API reference for OpenCV's C++ API and its Python API. When you look up a class or function, be sure to read the section under the heading Python.
OpenCV's Python module is named cv2. The 2 in cv2 has nothing to do with the version number of OpenCV; we really are using OpenCV 4. Historically, there was a cv Python module that wrapped a now-obsolete C version of OpenCV. The cv module does not exist anymore in OpenCV 4. However, the OpenCV documentation sometimes erroneously refers to the module name as cv instead...