add docs for the new attributes

crates/bevy_ecs/src/system/system_param.rs

@@ -43,7 +43,7 @@ use std::{
 /// will be created with the default value upon realisation.
 /// This is most useful for `PhantomData` fields, such as markers for generic types.
 ///
-/// # Example
+/// ### Example
 ///
 /// ```
 /// # use bevy_ecs::prelude::*;
@@ -66,6 +66,29 @@ use std::{
 /// # bevy_ecs::system::assert_is_system(my_system::<()>);
 /// ```
 ///
+/// `#[system_param(read_only)]` and `#[system_param(mutable)]`:
+/// Can be placed on the struct definition to indicate whether or not the type should
+/// implement [`ReadOnlySystemParam`].
+/// In most cases you do not need to worry about either of these attributes, since derived
+/// `SystemParam`s automatically implement `ReadOnlySystemParam` if each field does.
+///
+/// However, it is necessary to manually specifify this when you need to encapsulate
+/// private fields, since the default automagic `ReadOnlySystemParam` impl publicly
+/// exposes the type of each field.
+///
+/// ### Example
+///
+/// ```
+/// # use bevy_ecs::prelude::*;
+/// # use bevy_ecs_macros::SystemParam;
+/// #[derive(Resource)]
+/// struct MyResource(u32);
+///
+/// #[derive(SystemParam)]
+/// #[system_param(mutable)] // `MyResource` gets leaked if you don't add this
+/// pub struct MyParam<'w>(ResMut<'w, MyResource>);
+/// ```
+///
 /// # Generic `SystemParam`s
 ///
 /// When using the derive macro, you may see an error in the form of: