DynamicScene missing docs for entity_map arg in methods

## How can Bevy's documentation be improved?

functions `write_to_world` and [`write_to_world_with`](https://docs.rs/bevy/latest/bevy/prelude/struct.DynamicScene.html#method.write_to_world_with) are missing documentation describing the `entity_map` arg and its usage.

I think in my use case I can leave it empty? but I'm not sure what the intended usage is even after skimming through the source code.
Alternatively, create a newtype and document its usage there.

Start with the DynamicScene methods write_to_world and write_to_world_with, then inspect their source and the existing API documentation around entity_map. The documentation is done when it explains the argument's intended usage and clarifies whether an empty map is valid.

Written by the indexing model from the issue text.