Need documentation explaining how to handle snapshots

[todofixthis]

After a snapshot, previous transactions are lost. This means that methods such as get_account_data, get_new_addresses, get_transfers, etc. can no longer identify "used" addresses correctly.

At this time, the only way to properly identify "pre-snapshot" addresses is to keep track of them before the snapshot, then be sure to set the correct starting index (for key generation) when calling API methods.

1. Determine which API commands are impacted by snapshots (basically anything that relies on iter_used_addresses).
2. Update the documentation for each of these API commands with more info about how they are impacted by snapshots.