Document backing store

Is your feature request related to a problem? Please describe.
When I first used this SDK, I modified objects that were returned but just for the needs of my script, with no intention of pushing back to the API the modifications. It didn't work properly and I had a couple of issues that I didn't understand at first, until I noticed that this SDK uses a notion of "backing store". I didn't dig deeper what it was but now I understand thanks to this description in the Java Graph SDK for the "backing store" and "dirty" mechanisms which I believe are the same as in this Python SDK.

Describe the solution you'd like
If it's indeed the same thing, please raise awareness about it in the README (or elsewhere) to reduce confusion

Describe alternatives you've considered
Perhaps I could have spent more time and managed to find the article myself, or this https://learn.microsoft.com/en-us/openapi/kiota/models#backing-store

查看 README 以及链接的 Microsoft Graph Java 和 Kiota backing store 说明。确认 Python SDK 的 backing store 和 dirty mechanisms 的行为，然后在 README 或其他适当的位置记录这些内容，以便用户了解何时会跟踪返回对象的更改，或何时会将这些更改发送回 API。