Event snapshotting is confusing

https://github.com/matrix-org/matrix-js-sdk/pull/1738 introduces snapshots, but between the docs trying to explain that not everything is cloned and the fact that not everything is cloned, it's confusing. Part of this is due to technical overhead of trying to maintain rich models, so we may want to consider less dynamic models.

Start by reading PR #1738 and the snapshot documentation it introduced, then identify which parts of event models are and are not cloned. The issue does not name files or tests; work is not ready to call done until the intended snapshot behavior and model approach are agreed and the documentation explains that behavior clearly.

Written by the indexing model from the issue text.