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I sympathise with your confusion at the state of the official documentation, but your example is an issue with your Python syntax, not with OpenCV or its documentation. Importing like so:

import cv

will not add symbols in the cv module to the local namespace. You can either reference the cv module every time you want to use a symbol from it (e.g.: cv.cvtColor and cv.CV_RGB2GRAY) or import the symbols you need into the local namespace (e.g.: from cv import cvtColor, CV_RGB2GRAY) or just import everything: from cv import *.

I don't know of any better source of documentation, sorry.