A couple wording comments:

"All streams are careful about the type of data you give to them"
would read better as "All streams accept specific types of data".

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

Finally, not specific to this change, but I wonder if rather than
having distinct io.StringIO and io.BytesIO classes it would be better
to have a single io.MemoryIO class which takes mode arguments just
like io.FileIO?  The correspondence between file-based and memory-
based i/o would be more one-to-one.  Such a class could be added
without breaking existing code by using the StringIO and BytesIO
classes as the back-end for a MemoryIO class.