I wasn't able to find complete documentation for OpenCV's Python API anywhere, so I used pydoc to generate it.
The result is on GitHub Pages here.
pydoc generates very dated markup (<font>, <big>...), so I modified the pydoc script to generate (slightly) more semantic markup.
The output is still very poor compared to proper documentation (crammed onto a single page, many links broken), but it's good enough to ctrl + f through for method signatures.