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The documentation looks outdated, perhaps it predates the public C API.

docs.python.org/3/library/exceptions.html#SystemError の SystemError セクションから始め、C API の不適切な使用に関する issue の説明を確認してください。記載されている現在の実践を反映するようにドキュメントを更新し、拡張機能で検出可能な不正使用も含めてください。また、結果のテキストが CPython 内部エラーだけが原因であると示唆しないようにしてください。