fixed GPU minMaxLoc test, updated docs

doc/gpu_matrix_reductions.tex

@@ -89,7 +89,7 @@ void minMax(const GpuMat\& src, double* minVal, double* maxVal,\par
 \cvarg{src}{Single-channel source image.}
 \cvarg{minVal}{Pointer to returned minimum value. \texttt{NULL} if not required.}
 \cvarg{maxVal}{Pointer to returned maximum value. \texttt{NULL} if not required.}
-\cvarg{mask}{Optional mask to select a sub-matrix.}
+\cvarg{mask}{Optional mask to select a sub-matrix. Please note the result is undefined in the case of empty mask.}
 \cvarg{buf}{Optional buffer to avoid extra memory allocations. It's resized automatically.}
 \end{description}
 
@@ -112,7 +112,7 @@ void minMaxLoc(const GpuMat\& src, double* minVal, double* maxVal,\par
 \cvarg{maxVal}{Pointer to returned maximum value. \texttt{NULL} if not required.}
 \cvarg{minValLoc}{Pointer to returned minimum location. \texttt{NULL} if not required.}
 \cvarg{maxValLoc}{Pointer to returned maximum location. \texttt{NULL} if not required.}
-\cvarg{mask}{Optional mask to select a sub-matrix.}
+\cvarg{mask}{Optional mask to select a sub-matrix. Please note the result is undefined in the case of empty mask.}
 \cvarg{valbuf}{Optional values buffer to avoid extra memory allocations. It's resized automatically.}
 \cvarg{locbuf}{Optional locations buffer to avoid extra memory allocations. It's resized automatically.}
 \end{description}
@@ -132,4 +132,4 @@ int countNonZero(const GpuMat\& src, GpuMat\& buf);}
 \end{description}
 
 Function doesn't work with \texttt{CV\_64F} images on GPU with compute capability $<$ 1.3.\newline
-See also: \cvCppCross{countNonZero}.
+See also: \cvCppCross{countNonZero}.