Add note about solvePnP methods that cannot be used.

Add note: SOLVEPNP_EPNP will be used instead of SOLVEPNP_DLS and instead of SOLVEPNP_UPNP.

modules/calib3d/include/opencv2/calib3d.hpp

@@ -574,6 +574,9 @@ projections, as well as the camera matrix and the distortion coefficients.
         - Thus, given some data D = np.array(...) where D.shape = (N,M), in order to use a subset of
         it as, e.g., imagePoints, one must effectively copy it into a new array: imagePoints =
         np.ascontiguousarray(D[:,:2]).reshape((N,1,2))
+   -   The methods **SOLVEPNP_DLS** and **SOLVEPNP_UPNP** cannot be used as the current implementations are
+       unstable and sometimes give completly wrong results. If you pass one of these two flags,
+       **SOLVEPNP_EPNP** method will be used instead.
  */
 CV_EXPORTS_W bool solvePnP( InputArray objectPoints, InputArray imagePoints,
                             InputArray cameraMatrix, InputArray distCoeffs,