message: Impove the documentation for Mat::diag
diff: @@ -1085,6 +1085,29 @@ public:
  immediately below the main one.
  - `d>0` is a diagonal from the upper half. For example, d=1 means the diagonal is set
  immediately above the main one.
  + For example:
  + @code
  + Mat m = (Mat_<int>(3,3) <<
  + 1,2,3,
  + 4,5,6,
  + 7,8,9);
  + Mat d0 = m.diag(0);
  + Mat d1 = m.diag(1);
  + Mat d_1 = m.diag(-1);
  + @endcode
  + The resulting matrices are
  + @code
  + d0 =
  + [1;
  + 5;
  + 9]
  + d1 =
  + [2;
  + 6]
  + d_1 =
  + [4;
  + 8]
  + @endcode
  */
  Mat diag(int d=0) const;
  