Document the current practice for SystemError

@malemburg pointed that the documentation of SystemError only refers to CPython internal errors as the source.

https://docs.python.org/3/library/exceptions.html#SystemError

On practice, SystemError is also raised on improper use of the C API (like NULL pointer, or negative size, or wrong type in the concrete API) if it is possible to detect such error and it is not too costly (macros usually lack any argument checks). An alternative to SystemError is a crash or memory corruption. Currently the main source of SystemError is errors in third-party extensions, not in the Python core or standard extensions.

The documentation looks outdated, perhaps it predates the public C API.

從 docs.python.org/3/library/exceptions.html#SystemError 中的 SystemError 章節開始，檢視 issue 對不當使用 C API 的描述。更新文件以反映所述的目前實務，包括擴充功能中可偵測的誤用，並確保最終文字不再暗示 CPython 內部錯誤是唯一來源。