[docs] [issue37846] declare that Text I/O use buffer inside

[Windson Yang]

Windson Yang <wiwindson at outlook.com> added the comment:

I found the document is not that clear when I try to understand what happens when Python read/write a file. I'm not sure who also needs this information. As you said, It wouldn't help the user program in Python. However, make it more clear maybe help users have a better feeling of what is happening under the hood.

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue37846>
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