Add example data types for calibrateCamera call

It took me a while to figure out what was meant with

    OpenCV Error: Assertion failed (i < 0) in getMat

While searching for this error message I found [a list of error
messages](https://adventuresandwhathaveyou.wordpress.com/2014/03/14/opencv-error-messages-suck/)
which also explained what the problem was: The data type for `rvecs` was
not a simple `cv::Mat` but a `std::vector<cv::Mat>`.

After I fixed that, I got the next error message:

    OpenCV Error: Assertion failed (ni > 0 && ni == ni1) in
    collectCalibrationData, file
    /build/buildd/opencv-2.4.9+dfsg/modules/calib3d/src/calibration.cpp,
    line 3193

The problem here was that my data type for the `objectPoints` was just
`vector<Vec3f>` and not `vector<vector<Vec3f>>`.

In order to save other people the time looking for this, I added
explicit examples of the needed data types into the documentation of the
function. I had to re-read the current version a couple of times until I
can read the needed levels of `vector<>`. Having this example would have
really helped me there.

modules/calib3d/include/opencv2/calib3d.hpp

@@ -697,19 +697,19 @@ CV_EXPORTS_W bool findCirclesGrid( InputArray image, Size patternSize,
 
 /** @brief Finds the camera intrinsic and extrinsic parameters from several views of a calibration pattern.
 
-@param objectPoints In the new interface it is a vector of vectors of calibration pattern points
-in the calibration pattern coordinate space. The outer vector contains as many elements as the
-number of the pattern views. If the same calibration pattern is shown in each view and it is fully
-visible, all the vectors will be the same. Although, it is possible to use partially occluded
-patterns, or even different patterns in different views. Then, the vectors will be different. The
-points are 3D, but since they are in a pattern coordinate system, then, if the rig is planar, it
-may make sense to put the model to a XY coordinate plane so that Z-coordinate of each input object
-point is 0.
+@param objectPoints In the new interface it is a vector of vectors of calibration pattern points in
+the calibration pattern coordinate space (e.g. std::vector<std::vector<cv::Vec3f>>). The outer
+vector contains as many elements as the number of the pattern views. If the same calibration pattern
+is shown in each view and it is fully visible, all the vectors will be the same. Although, it is
+possible to use partially occluded patterns, or even different patterns in different views. Then,
+the vectors will be different. The points are 3D, but since they are in a pattern coordinate system,
+then, if the rig is planar, it may make sense to put the model to a XY coordinate plane so that
+Z-coordinate of each input object point is 0.
 In the old interface all the vectors of object points from different views are concatenated
 together.
-@param imagePoints In the new interface it is a vector of vectors of the projections of
-calibration pattern points. imagePoints.size() and objectPoints.size() and imagePoints[i].size()
-must be equal to objectPoints[i].size() for each i.
+@param imagePoints In the new interface it is a vector of vectors of the projections of calibration
+pattern points (e.g. std::vector<std::vector<cv::Vec2f>>). imagePoints.size() and
+objectPoints.size() and imagePoints[i].size() must be equal to objectPoints[i].size() for each i.
 In the old interface all the vectors of object points from different views are concatenated
 together.
 @param imageSize Size of the image used only to initialize the intrinsic camera matrix.
@@ -719,11 +719,11 @@ and/or CV_CALIB_FIX_ASPECT_RATIO are specified, some or all of fx, fy, cx, cy mu
 initialized before calling the function.
 @param distCoeffs Output vector of distortion coefficients
 \f$(k_1, k_2, p_1, p_2[, k_3[, k_4, k_5, k_6],[s_1, s_2, s_3, s_4]])\f$ of 4, 5, 8 or 12 elements.
-@param rvecs Output vector of rotation vectors (see Rodrigues ) estimated for each pattern view.
-That is, each k-th rotation vector together with the corresponding k-th translation vector (see
-the next output parameter description) brings the calibration pattern from the model coordinate
-space (in which object points are specified) to the world coordinate space, that is, a real
-position of the calibration pattern in the k-th pattern view (k=0.. *M* -1).
+@param rvecs Output vector of rotation vectors (see Rodrigues ) estimated for each pattern view
+(e.g. std::vector<cv::Mat>>). That is, each k-th rotation vector together with the corresponding
+k-th translation vector (see the next output parameter description) brings the calibration pattern
+from the model coordinate space (in which object points are specified) to the world coordinate
+space, that is, a real position of the calibration pattern in the k-th pattern view (k=0.. *M* -1).
 @param tvecs Output vector of translation vectors estimated for each pattern view.
 @param flags Different flags that may be zero or a combination of the following values:
 -   **CV_CALIB_USE_INTRINSIC_GUESS** cameraMatrix contains valid initial values of