Improve `Assests` .get() docs

## How can Bevy's documentation be improved?
https://github.com/bevyengine/bevy/blob/9309d89bb84c9e9708c0061fb495af104063308b/crates/bevy_asset/src/assets.rs#L133-L139

It's unclear to me in which cases the result would be None:
- Is the asset not finished loading?
- Is the handle invalid in some way?
- Something completely different?

Thanks!

Read the documentation for `Assets::get` in `crates/bevy_asset/src/assets.rs` at the linked lines. Trace the surrounding asset lookup behavior to determine the cases that produce `None`, then update the documentation to explain those cases clearly and verify the wording against the implementation.

Written by the indexing model from the issue text.