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 suggest to revert removing `namedWindow ` line. You modified the description of the function `imshow` and I guess you also affected the header of the functions below, which is `namedWindow`. You added the symbol `.` after its name.