Improve the doc for fundamental matrix.

modules/core/include/opencv2/core.hpp

@@ -1894,6 +1894,7 @@ The function solveCubic finds the real roots of a cubic equation:
 The roots are stored in the roots array.
 @param coeffs equation coefficients, an array of 3 or 4 elements.
 @param roots output array of real roots that has 1 or 3 elements.
+@return number of real roots. It can be 0, 1 or 2.
 */
 CV_EXPORTS_W int solveCubic(InputArray coeffs, OutputArray roots);