Watching entity after spawning an observer can contribute to unnecessary bugs

The observer function [watch_entity](https://docs.rs/bevy/latest/bevy/ecs/prelude/struct.Observer.html#method.watch_entity) documentation states that watching entities after observer has been spawned will produce no effects. In some situations this may lead to unnecessary and unintended bugs if the instance of the observer is being kept alive and used to watch entities beyond next world synchronization point. If possible, is there a reason not to panic in this situation to make sure the user realizes that watching an entity with spawned observer instance has no effect?

Start with the linked Observer::watch_entity documentation and inspect the observer lifecycle around watching entities after spawning. Determine whether this state should panic or remain a documented no-op; done means the behavior and its rationale are clear to callers.

Written by the indexing model from the issue text.