Small Docs PR for Deferred Worlds (#18384)

# Objective

I was looking over a PR yesterday, and got confused by the docs on
deferred world. I thought I would add a little more detail to the struct
in order to clarify it a little.

## Solution

Document some more about deferred worlds.

crates/bevy_ecs/src/world/deferred_world.rs

@@ -20,6 +20,8 @@ use super::{unsafe_world_cell::UnsafeWorldCell, Mut, World, ON_INSERT, ON_REPLAC
 
 /// A [`World`] reference that disallows structural ECS changes.
 /// This includes initializing resources, registering components or spawning entities.
+///
+/// This means that in order to add entities, for example, you will need to use commands instead of the world directly.
 pub struct DeferredWorld<'w> {
     // SAFETY: Implementors must not use this reference to make structural changes
     world: UnsafeWorldCell<'w>,