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Agree, it should be documented properly.
I've added _asyncio_internal flag.
Also, asincio.__init__ has __getattr__() now to raise DeprecationWarning for importing StreamReader, StreamWriter and StreamReaderProtocol.
We have an agreement with @1st1 to land asyncio streams code changes first and document them later before Release Candidate stage.

I supposed to make a PR for documentation. If somebody would help me -- I very appreciate this.
Especially taking into account that my English is rough :(
Usually Yuri replaces almost every my sentence on review :)