[docs] [issue20699] Document that binary IO classes work with bytes-likes objects

[Martin Panter]

Martin Panter added the comment:

Posting patch v3; thanks for the reviews!

* Changed “buffer” → “object”
* Made it clearer that the bytes-like object generalization only applies to method arguments, not return values
* Minor fixes to wording

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Added file: http://bugs.python.org/file38181/bytes-like-param.v3.patch

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