"newline controls how universal newlines works (it only applies to text mode). It can be None, '', '\n', '\r', and '\r\n'..."

Whereas splitlines says:

"Return a list of the lines in the string, breaking at line boundaries."

So if we are fixing docs, we need to add that "line boundaries" are based on the relevant unicode properties (see issue 7643).

And, indeed, Antoine has already pronounced on this in issue 6664.

Since this has come up more than once, adding a note that they are not recognized by design in the io module to the io docs might be a good idea.