Document options for !Sync types for Component and Resources (#6864)

# Objective
It's not clear to users how to handle `!Sync` types as components and resources in the absence of engine level support for them. 

## Solution
Added a section to `Component`'s and `Resource`'s type level docs on available options for making a type `Sync` when it holds `!Sync` fields, linking `bevy_utils::synccell::SyncCell` and the currently unstable `std::sync::Exclusive`.

Also added a compile_fail doctest that illustrates how to apply `SyncCell`. These will break when/if #6572 gets merged, at which point these docs should be updated.

crates/bevy_ecs/src/component.rs

@@ -110,6 +110,38 @@ use std::{
 ///
 /// [orphan rule]: https://doc.rust-lang.org/book/ch10-02-traits.html#implementing-a-trait-on-a-type
 /// [newtype pattern]: https://doc.rust-lang.org/book/ch19-03-advanced-traits.html#using-the-newtype-pattern-to-implement-external-traits-on-external-types
+///
+/// # `!Sync` Components
+/// A `!Sync` type cannot implement `Component`. However, it is possible to wrap a `Send` but not `Sync`
+/// type in [`SyncCell`] or the currently unstable [`Exclusive`] to make it `Sync`. This forces only
+/// having mutable access (`&mut T` only, never `&T`), but makes it safe to reference across multiple
+/// threads.
+///
+/// This will fail to compile since `RefCell` is `!Sync`.
+/// ```compile_fail
+/// # use std::cell::RefCell;
+/// # use bevy_ecs::component::Component;
+/// #[derive(Component)]
+/// struct NotSync {
+///    counter: RefCell<usize>,
+/// }
+/// ```
+///
+/// This will compile since the `RefCell` is wrapped with `SyncCell`.
+/// ```
+/// # use std::cell::RefCell;
+/// # use bevy_ecs::component::Component;
+/// use bevy_utils::synccell::SyncCell;
+///
+/// // This will compile.
+/// #[derive(Component)]
+/// struct ActuallySync {
+///    counter: SyncCell<RefCell<usize>>,
+/// }
+/// ```
+///
+/// [`SyncCell`]: bevy_utils::synccell::SyncCell
+/// [`Exclusive`]: https://doc.rust-lang.org/nightly/std/sync/struct.Exclusive.html
 pub trait Component: Send + Sync + 'static {
     type Storage: ComponentStorage;
 }

crates/bevy_ecs/src/system/system_param.rs

@@ -89,6 +89,38 @@ use std::{
 ///
 /// This will most commonly occur when working with `SystemParam`s generically, as the requirement
 /// has not been proven to the compiler.
+///
+/// # `!Sync` Resources
+/// A `!Sync` type cannot implement `Resource`. However, it is possible to wrap a `Send` but not `Sync`
+/// type in [`SyncCell`] or the currently unstable [`Exclusive`] to make it `Sync`. This forces only
+/// having mutable access (`&mut T` only, never `&T`), but makes it safe to reference across multiple
+/// threads.
+///
+/// This will fail to compile since `RefCell` is `!Sync`.
+/// ```compile_fail
+/// # use std::cell::RefCell;
+/// # use bevy_ecs::system::Resource;
+///
+/// #[derive(Resource)]
+/// struct NotSync {
+///    counter: RefCell<usize>,
+/// }
+/// ```
+///
+/// This will compile since the `RefCell` is wrapped with `SyncCell`.
+/// ```
+/// # use std::cell::RefCell;
+/// # use bevy_ecs::system::Resource;
+/// use bevy_utils::synccell::SyncCell;
+///
+/// #[derive(Resource)]
+/// struct ActuallySync {
+///    counter: SyncCell<RefCell<usize>>,
+/// }
+/// ```
+///
+/// [`SyncCell`]: bevy_utils::synccell::SyncCell
+/// [`Exclusive`]: https://doc.rust-lang.org/nightly/std/sync/struct.Exclusive.html
 pub trait SystemParam: Sized {
     type Fetch: for<'w, 's> SystemParamFetch<'w, 's>;
 }