avoid collapsing exception groups from user code

[graingert]

Summary

If user code is raising ExceptionGroups eg from TaskGroups or otherwise, they should get ExceptionGroups raised. There could also be important notes attached.

Checklist

• [x] I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• [x] I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• [x] I've updated the documentation accordingly.

No docs changes needed, probably should have made an issue for this.

[hasB4K]

I have encountered this issue with starlette. I would love to have this PR merged tbh :) Amazing work