Add usage notes for the IntoX family of ECS traits (#17379)

# Objective

Occasionally bevy users will want to store systems or observer systems
in a component or resource, but they first try to store `IntoSystem`
instead of `System`, which leads to some headaches having to deal with
the `M` marker type parameter. We should recommend they use the `X`
trait instead of the `IntoX` trait in that case, as well for returning
from a function.

## Solution

Add usage notes to the `IntoX` traits about using `X` instead.

Author: ItsDoot

crates/bevy_ecs/src/schedule/config.rs

@@ -247,6 +247,12 @@ impl<T> NodeConfigs<T> {
 /// [`SystemParam`](crate::system::SystemParam)), or tuples thereof.
 /// It is a common entry point for system configurations.
 ///
+/// # Usage notes
+///
+/// This trait should only be used as a bound for trait implementations or as an
+/// argument to a function. If system configs need to be returned from a
+/// function or stored somewhere, use [`SystemConfigs`] instead of this trait.
+///
 /// # Examples
 ///
 /// ```
@@ -617,6 +623,12 @@ impl SystemSetConfig {
 pub type SystemSetConfigs = NodeConfigs<InternedSystemSet>;
 
 /// Types that can convert into a [`SystemSetConfigs`].
+///
+/// # Usage notes
+///
+/// This trait should only be used as a bound for trait implementations or as an
+/// argument to a function. If system set configs need to be returned from a
+/// function or stored somewhere, use [`SystemSetConfigs`] instead of this trait.
 #[diagnostic::on_unimplemented(
     message = "`{Self}` does not describe a valid system set configuration",
     label = "invalid system set configuration"

crates/bevy_ecs/src/schedule/set.rs

@@ -152,6 +152,12 @@ impl SystemSet for AnonymousSet {
 }
 
 /// Types that can be converted into a [`SystemSet`].
+///
+/// # Usage notes
+///
+/// This trait should only be used as a bound for trait implementations or as an
+/// argument to a function. If a system set needs to be returned from a function
+/// or stored somewhere, use [`SystemSet`] instead of this trait.
 #[diagnostic::on_unimplemented(
     message = "`{Self}` is not a system set",
     label = "invalid system set"

crates/bevy_ecs/src/system/mod.rs

@@ -160,6 +160,12 @@ use crate::world::World;
 /// Use this to get a system from a function. Also note that every system implements this trait as
 /// well.
 ///
+/// # Usage notes
+///
+/// This trait should only be used as a bound for trait implementations or as an
+/// argument to a function. If a system needs to be returned from a function or
+/// stored somewhere, use [`System`] instead of this trait.
+///
 /// # Examples
 ///
 /// ```

crates/bevy_ecs/src/system/observer_system.rs

@@ -21,6 +21,12 @@ impl<
 }
 
 /// Implemented for systems that convert into [`ObserverSystem`].
+///
+/// # Usage notes
+///
+/// This trait should only be used as a bound for trait implementations or as an
+/// argument to a function. If an observer system needs to be returned from a
+/// function or stored somewhere, use [`ObserverSystem`] instead of this trait.
 #[diagnostic::on_unimplemented(
     message = "`{Self}` cannot become an `ObserverSystem`",
     label = "the trait `IntoObserverSystem` is not implemented",