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The documentation looks outdated, perhaps it predates the public C API.

从 docs.python.org/3/library/exceptions.html#SystemError 中的 SystemError 部分开始，审阅 issue 对不当使用 C API 的描述。更新文档以反映所述的当前实践，包括扩展中的可检测误用，并确保最终文本不再暗示 CPython 内部错误是唯一来源。