Move default docs (#9638)

# Objective

- Make the default docs more useful like suggested in
#9600 (comment)

## Solution

- Move the documentation to the `fn default()` method instead of the
`impl Default`.

Allows you to view the docs directly on the function without having to
go to the implementation.

### Before
![Screenshot 2023-08-29 at 18 21
03](https://github.com/bevyengine/bevy/assets/104745335/6d31591e-f190-4b8e-8bc3-a570ada294f0)

### After
![Screenshot 2023-08-29 at 18 19
54](https://github.com/bevyengine/bevy/assets/104745335/e2442ec1-593d-47f3-b539-8c77a170f0b6)

crates/bevy_ecs/src/schedule/schedule.rs

@@ -168,11 +168,11 @@ pub struct Schedule {
 #[derive(ScheduleLabel, Hash, PartialEq, Eq, Debug, Clone)]
 struct DefaultSchedule;
 
-/// Creates a schedule with a default label. Only use in situations where
-/// where you don't care about the [`ScheduleLabel`]. Inserting a default schedule
-/// into the world risks overwriting another schedule. For most situations you should use
-/// [`Schedule::new`].
 impl Default for Schedule {
+    /// Creates a schedule with a default label. Only use in situations where
+    /// you don't care about the [`ScheduleLabel`]. Inserting a default schedule
+    /// into the world risks overwriting another schedule. For most situations
+    /// you should use [`Schedule::new`].
     fn default() -> Self {
         Self::new(DefaultSchedule)
     }

crates/bevy_render/src/view/visibility/render_layers.rs

@@ -38,8 +38,8 @@ impl std::iter::FromIterator<Layer> for RenderLayers {
     }
 }
 
-/// Defaults to containing to layer `0`, the first layer.
 impl Default for RenderLayers {
+    /// By default, this structure includes layer `0`, which represents the first layer.
     fn default() -> Self {
         RenderLayers::layer(0)
     }