Document migration from cache-based sync to the fully stateless sync

[jannden]

Before famedly-sync 0.8, we used a "cache" to identify what changes were made to the ldap users. This caused many issues, and we migrated away from this.

That, however, required using a new encoding for user IDs. We've written a migration script, but this is untested and undocumented. We need to test a full migration and then document the steps involved.

This has already caused some issues in production, since user localparts change and users get deleted (without new users being imported); so far this generally causes breakage.

See the parent task and the main PR with it's comments.