calib3d: improve documentation of reprojectImageTo3D

Make a note about 16-bit signed format - the function assumes that
values have no fractional bits (so 16-bit disparity from StereoBM
and StereoSGBM cannot be directly used!)

modules/calib3d/include/opencv2/calib3d.hpp

@@ -1375,7 +1375,8 @@ CV_EXPORTS_W void validateDisparity( InputOutputArray disparity, InputArray cost
 /** @brief Reprojects a disparity image to 3D space.
 
 @param disparity Input single-channel 8-bit unsigned, 16-bit signed, 32-bit signed or 32-bit
-floating-point disparity image.
+floating-point disparity image. If 16-bit signed format is used, the values are assumed to have no
+fractional bits.
 @param _3dImage Output 3-channel floating-point image of the same size as disparity . Each
 element of _3dImage(x,y) contains 3D coordinates of the point (x,y) computed from the disparity
 map.