OpenCV's documentation is online, at https://docs.opencv.org/master/. The documentation includes a combined API reference for the latest OpenCV C++ API and its latest Python API (which is based on the C++ API). The latest Java API documentation is online, at http://docs.opencv.org/master/javadoc/.
If the documentation seems to leave your questions unanswered, try reaching out to the OpenCV community instead. The following sites are good venues for questions, answers, and shared experience:
- The official OpenCV forum, at http://answers.opencv.org/questions/
- The PyImageSearch site, where Adrian Rosebrock teaches computer vision and machine learning, at https://www.pyimagesearch.com/
- The support site for my OpenCV books, at http://nummist.com/opencv/
Finally, if you are an advanced user who wants to try new features, bug-fixes...