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since opencv3, the python docs are no more generated, the last version, that had those is this one

but you can use the builtin help function:

>>> help(cv2)

to see all of it, or in your case:

>>> help(cv2.HoughLinesP)
Help on built-in function HoughLinesP:

HoughLinesP(...)
    HoughLinesP(image, rho, theta, threshold[, lines[, minLineLength[, maxLineGap]]]) -> lines

>>>