Fix#3714:Improve Display Image Tutorial

For Bug #3714, user use the imshow() function without calling
namedWindow() properly, because user did not know the implicit rule for
imshow(). (i.e. without calling namedWindow implied calling namedWindow(
,CV__WINDOW_AUTOSIZE).

Improve the Display Image Tutorial to educate the user for such implied
rule.

Put a note on documentation for Bug#3714

Revise the documentation regarding to imshow() function to better
educate the user the implicit rule. (i.e. without creating the window
before the imshow() function is the same as creating AUTOSIZE window)

change back to new variables