Fix#3714:Improve Display Image Tutorial

Changing both the documentation and the tutorial to better educate the users regarding to the implicit rule in imshow() function. (i.e. without creating the window before the imshow() function is the same as creating AUTOSIZE window)

For bug information, please check this link:http://code.opencv.org/issues/3714
For previous pull request comment, please refer to pull request #3441,#3455
I changed this line of documentation because it is not explicitly said in the documentation before that namedWindow(..,CV_WINDOW_AUTOSIZE) is assumed if user hadn't called namedWindow before imshow(). That's why the user misuse the code and report the bug in Bug#3714.So I am wondering why @ilya-lavrenov suggest to revert this change