add doc comments for FunctionSystem and distributive_run_if

crates/bevy_ecs/src/schedule/config.rs

@@ -394,8 +394,8 @@ where
 
     /// Add a run condition to each contained system.
     ///
-    /// Each system will receive its own clone of the [`Condition`] and will only run
-    /// if the `Condition` is true.
+    /// The [`Condition`] must be [`Clone`]. Each system will receive its own clone
+    /// of the `Condition` and will only run if the `Condition` is true.
     ///
     /// Each individual condition will be evaluated at most once (per schedule run),
     /// right before the corresponding system prepares to run.
@@ -422,6 +422,9 @@ where
     /// Use [`run_if`](IntoSystemSetConfig::run_if) on a [`SystemSet`] if you want to make sure
     /// that either all or none of the systems are run, or you don't want to evaluate the run
     /// condition for each contained system separately.
+    ///
+    /// The [`Condition`] is cloned for each system.
+    /// Cloned instances of [`FunctionSystem`](crate::system::FunctionSystem) will be de-initialized.
     fn distributive_run_if<M>(self, condition: impl Condition<M> + Clone) -> SystemConfigs {
         self.into_configs().distributive_run_if(condition)
     }

crates/bevy_ecs/src/system/function_system.rs

@@ -367,6 +367,9 @@ pub struct In<In>(pub In);
 /// becomes the functions [`In`] tagged parameter or `()` if no such parameter exists.
 ///
 /// [`FunctionSystem`] must be `.initialized` before they can be run.
+///
+/// The [`Clone`] implementation for [`FunctionSystem`] returns a new instance which
+/// is NOT initialized. The cloned system must also be `.initialized` before it can be run.
 pub struct FunctionSystem<Marker, F>
 where
     F: SystemParamFunction<Marker>,