Added documentation on the query filters Mutated<T> Changed<T> and Added<T>

This documents both the non-obvious interaction with non-explicit system ordering
and adds examples for Changed and Added. This is in response to #1551

Author: sdfgeoff

crates/bevy_ecs/src/core/filter.rs

@@ -31,12 +31,52 @@ pub struct Or<T>(pub T);
 
 /// Query transformer that retrieves components of type `T` that have been mutated since the start of the frame.
 /// Added components do not count as mutated.
+/// This filter is useful as a performance optimization as it means that a system can be made to run only when
+/// it needs to rather than every frame.
+///
+/// Because the ordering of systems can change and this filter is only effective on changes before the query executes
+/// you need to use explicit dependency ordering or ordered stages for these query filters to be useful.
+///
+/// Example:
+/// ```
+/// use bevy::prelude::*;
+/// 
+/// fn print_moving_objects_system(query: Query<(Name,), (Mutated<Transform>,)>) {
+///     for (name, ) in query.iter() {
+///         println!("Entity Moved: {}", name)
+///     }
+/// }
+/// ```
 pub struct Mutated<T>(NonNull<ComponentFlags>, PhantomData<T>);
 
 /// Query transformer that retrieves components of type `T` that have been added since the start of the frame.
+/// This filter is useful as a performance optimization as it means that a system can be made to run only when
+/// it needs to rather than every frame.
+///
+/// Because the ordering of systems can change and this filter is only effective on changes before the query executes
+/// you need to use explicit dependency ordering or ordered stages for these query filters to be useful.
+///
+///
+/// Example:
+/// ```
+/// use bevy::prelude::*;
+/// 
+/// fn print_moving_objects_system(query: Query<(Name,), (Added<Name>,)>) {
+///     for (name, ) in query.iter() {
+///         println!("Named entity created: {}", name)
+///     }
+/// }
+/// ```
 pub struct Added<T>(NonNull<ComponentFlags>, PhantomData<T>);
 
 /// Query transformer that retrieves components of type `T` that have either been mutated or added since the start of the frame.
+/// This filter is useful as a performance optimization as it means that a system can be made to run only when
+/// it needs to rather than every frame.
+///
+/// Because the ordering of systems can change and this filter is only effective on changes before the query executes
+/// you need to use explicit dependency ordering or ordered stages for these query filters to be useful.
+/// 
+/// Also see the documentation for [`Mutated<T>`] and [`Added`] as this filter is a logical OR of them.
 pub struct Changed<T>(NonNull<ComponentFlags>, PhantomData<T>);
 
 impl QueryFilter for () {