Improve `AssetEvent` docs

The docs for [`AssetEvent`](https://docs.rs/bevy/latest/bevy/asset/prelude/enum.AssetEvent.html) are a bit vague on what exactly it means for an asset to be "added", or "modified" or "loaded with dependencies".

For instance, when hot-reloading an asset, we expect `Modified` to fire, but what about `LoadedWithDependencies`? Will that fire? What if the newly modified asset has additional dependencies?

It should be easy to deduce exactly when an asset event will fire for a particular sequence of asset operations.

Start with the AssetEvent API documentation linked in the issue and trace the documented asset operations, including hot-reloading and dependency changes. Clarify exactly when Added, Modified, and LoadedWithDependencies fire for each sequence, including modified dependencies, and update the docs so those outcomes are unambiguous.

Written by the indexing model from the issue text.