Mention world_query(ignore) attribute for WorldQuery derivation

Fixes #6283

crates/bevy_ecs/src/query/fetch.rs

@@ -55,7 +55,12 @@ use std::{cell::UnsafeCell, marker::PhantomData};
 /// - Methods can be implemented for the query items.
 /// - There is no hardcoded limit on the number of elements.
 ///
-/// This trait can only be derived if each field of the struct also implements `WorldQuery`.
+/// This trait can only be derived if each field either
+///
+/// * also implements `WorldQuery`, or
+/// * is marked with `#[world_query(ignore)]`. Fields decorated with this attribute
+///   will be created with the default value upon realisation.
+///
 /// The derive macro only supports regular structs (structs with named fields).
 ///
 /// ```

examples/ecs/custom_query_param.rs

@@ -81,6 +81,8 @@ struct CustomQuery<T: Component + Debug, P: Component + Debug> {
     optional_tuple: Option<(NestedQuery, &'static mut ComponentZ)>,
     generic: GenericQuery<T, P>,
     empty: EmptyQuery,
+    #[world_query(ignore)]
+    foo: String,
 }
 
 // This is a valid query as well, which would iterate over every entity.