Docs: fixed _dest type in cv::compare

modules/core/include/opencv2/core.hpp

@@ -1282,7 +1282,8 @@ equivalent matrix expressions:
 @endcode
 @param src1 first input array or a scalar; when it is an array, it must have a single channel.
 @param src2 second input array or a scalar; when it is an array, it must have a single channel.
-@param dst output array that has the same size and type as the input arrays.
+@param dst output array of type ref CV_8U that has the same size and the same number of channels as
+    the input arrays.
 @param cmpop a flag, that specifies correspondence between the arrays (cv::CmpTypes)
 @sa checkRange, min, max, threshold
 */