Clarify when time-varying CFR applies

It might be good to clarify (in the vignette / docs) when the time-varying CFR applies as it affects the interpretation. If it's due to e.g. a new variant or cases moving into different age groups this would apply at the time at which cases appear. If it's e.g. due to hospital capacity issues it's more likely to apply at the time of death. If I understand the methods correctly in `cfr_time_varying` the application is at the time of death.