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

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

array is supported explicitly in the Python implementation of readinto().

What if write not "usable with memoryview", but mention memoryview as an example?

   The object *b* should be a pre-allocated, writable array of bytes,
   such as :class:`bytearray`, :class:`array.array` or
   :class:`memoryview`.

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<http://bugs.python.org/issue20699>
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