update docs

Author: NathanSWard

crates/bevy_ecs/src/system/commands/fallible.rs

@@ -3,13 +3,15 @@ use crate::world::World;
 use bevy_utils::tracing::error;
 use std::fmt::Debug;
 
-/// TODO:
+/// The error struct provided to command error handlers.
+/// This contains both the error, and a mutable reference to [`World`].
 pub struct CommandError<'a, C: FallibleCommand> {
     pub error: C::Error,
     pub world: &'a mut World,
 }
 
-/// Used for specifying and error for a command.
+/// A [`World`] mutation that can potentially fail.
+/// For an infallible variant, use [`Command`].
 pub trait FallibleCommand: Send + Sync + 'static {
     type Error;