I would actually remove the whole section about readlines() or possibly just mention it briefly (something like "If you want to read all the lines of a file in a list you can also use f.readlines().")
The sizehint arg is rarely used, so I don't see the point of going in such details about it in the tutorial.  In Lib/, there are only a couple of places where it's actually used:
Lib/fileinput.py:358: self._buffer = self._file.readlines(self._bufsize)
Lib/idlelib/GrepDialog.py:90: block = f.readlines(100000)