[docs] [issue37465] Incorrect documentation for `s#` arguments in C API argument parsing

[Inada Naoki]

Inada Naoki <songofacandy at gmail.com> added the comment:

See note in https://docs.python.org/3.9/c-api/arg.html#strings-and-buffers

"""
Note: For all # variants of formats (s#, y#, etc.), the type of the length argument (int or Py_ssize_t) is controlled by defining the macro PY_SSIZE_T_CLEAN before including Python.h. If the macro was defined, length is a Py_ssize_t rather than an int. This behavior will change in a future Python version to only support Py_ssize_t and drop int support. It is best to always define PY_SSIZE_T_CLEAN. 
"""

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nosy: +inada.naoki

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