Bug3714 fix

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
It is strange that this file is in your commit history. I suggest doing the following:

1) git rm .gitignore
2) git commit -m "fix"
3) git rebase -i HEAD~2 --> remove the last message and squash two last commits
4) push your changes

This will fix this problem.