Updates to stereoRectifyUncalibrated

The first commit fixes a bug with the input argument validation for stereoRectifyUncalibrated.

It was throwing an error when used in the following way, which was specified in stereoRectifyUncalibrated.m:
[H1,H2] = cv.stereoRectifyUncalibrated(points1, points2, F, imgSize)

The second and third commits updates the documentation for stereoRectifyUncalibrated, pointing out that the input structure is different from that of findFundamentalMatrix, as seen here: http://opencv-users.1802565.n2.nabble.com/StereoRectifyUncalibrated-not-accepting-same-array-as-FindFundamentalMat-td5149185.html

The points must be in a vector of the form (x_1, y_1, x_2, y_2 ... x_n, y_n)