Improve doc of calcHist overload that accepts vectors

It's not clear how ranges argument should be used in the overload of
calcHist that accepts std::vector. The main overload uses array of
arrays there, while std::vector overload uses a plain array. The code
interprets the vector as a flattened array and rebuilds array of arrays
from it. This is not obvious interpretation, so documentation has been
added to explain the expected usage.

modules/imgproc/include/opencv2/imgproc.hpp

@@ -3270,7 +3270,14 @@ CV_EXPORTS void calcHist( const Mat* images, int nimages,
                           const int* histSize, const float** ranges,
                           bool uniform = true, bool accumulate = false );
 
-/** @overload */
+/** @overload
+
+this variant supports only uniform histograms.
+
+ranges argument is either empty vector or a flattened vector of histSize.size()*2 elements
+(histSize.size() element pairs). The first and second elements of each pair specify the lower and
+upper boundaries.
+*/
 CV_EXPORTS_W void calcHist( InputArrayOfArrays images,
                             const std::vector<int>& channels,
                             InputArray mask, OutputArray hist,