Clarified parameter useExtrinsicGuess in solvePnP

I reckon that the parameter useExtrinsicGuess is only used for the ITERATIVE method - [see solvepnp.cpp](https://github.com/PhilLab/opencv/blob/patch-2/modules/calib3d/src/solvepnp.cpp#L51-L99).
This PR clarifies the documentation.

*However*, I'm puzzled by the fact that rvec and tvec are of the type OutputArray - should this be changed to InputOutputArray?