Document BytesIO buffer size

[jxu]

https://stackoverflow.com/questions/26827055/python-how-to-get-bytesio-allocated-memory-length

The length of the data stored in a BytesIO object can be accessed in several ways. The best way seems to be getbuffer().nbytes. But it's not documented here
https://docs.python.org/3/library/io.html#io.BytesIO

Only here, the gigantic built-in types page https://docs.python.org/3/library/stdtypes.html#memoryview.nbytes

[StanFromIreland]

It is not undocumented as you said. Splitting up the gigantic page is tracked here: #126052