Migration guide: Mention `$state.snapshot` as potentially being required

[brunnerh]

Describe the problem

Third party APIs (and some built-ins like structuredClone) may be unable to deal with state proxies, this can cause errors/confusion.

The fact that proxies are created by $state and how that matters is not explained. (There is only a single mention of proxies in the "Modern browser required" section.)

Describe the proposed solution

• Explain that objects and arrays are proxied.
• Give an example where a proxy could cause an error and how $state.snapshot helps.

Importance

nice to have

[Ocean-OS]

This is part of what #14599 will do when it's finished.