[3.6] bpo-28647: Update -u documentation

[matrixise]

• stdout and stderr are always opened in text mode
• and they are line-buffered

Initial patch by Gareth Rees.

backport of #1655 for 3.6

[Mariatta]

Thanks for the PR @matrixise

I would prefer to wait that the PR on master be merged first, and the use the squashed commit hash on that PR to be cherry-picked (either manual git cherry-pick, or using cherry_picker.py

Reasons:

• if we need any changes made to the original PR, we don't need to update the backport PRs
• when we cherry-pick the squashed commit hash, the original PR gets linked to the backport PRs automatically.

Do you think you can help me with backporting GH-1306 to 2.7 please? I was supposed to do it but came across conflict and got distracted with PyCon 😅 Thank you! 😄

[Mariatta]

It was mentioned in #1655 that the backport is not yet needed. I'm closing this.
Thanks @matrixise