Document snapshot equality

[zgrannan]

Please feel free to suggest changes. Ideally, I'd like to explain the feature without requiring the audience to necessarily understand the snapshot encoding.

[Aurel300]

Looks good. Although we indeed might not want to explain snapshots fully (we've had this discussion a couple of times already!), we could at least note that snapshots follow nested fields and references, but not unsafe pointers or UnsafeCells.

[Aurel300]

Thanks! I'll merge this now since the documentation is helpful, we can adjust the wording later if anybody finds it confusing.