[issue9715] io doc improvements

[Antoine Pitrou]

Antoine Pitrou <pitrou at free.fr> added the comment:

> "The default mode is ``'r'`` (open for reading text, synonym of
> ``'rt'``)".  I liked the original wording better.

Well, people use "r" in practice, and "rt" is a somewhat rarer
alternative. We could drop the "synonym..." part entirely.

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