Documentation calib3d. R - same name for different matrix.

Perhaps renaming the matrix of rectification transformations will make [this formula](https://github.com/opencv/opencv/blob/4.1.2/modules/calib3d/include/opencv2/calib3d.hpp#L2887) more readable.
Also, [extra (](
https://github.com/opencv/opencv/blob/4.1.2/modules/calib3d/include/opencv2/calib3d.hpp#L2896)