Clarify the difference between default render layers and none render layers (#14075)

# Objective

It's not always obvious what the default value for `RenderLayers`
represents. It is documented, but since it's an implementation of a
trait method the documentation may or may not be shown depending on the
IDE.

## Solution

Add documentation to the `none` method that explicitly calls out the
difference.

---------

Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>

Author: joseph-gio

Cargo.toml

@@ -10,7 +10,7 @@ keywords = ["game", "engine", "gamedev", "graphics", "bevy"]
 license = "MIT OR Apache-2.0"
 repository = "https://github.com/bevyengine/bevy"
 documentation = "https://docs.rs/bevy"
-rust-version = "1.78.0"
+rust-version = "1.79.0"
 
 [workspace]
 exclude = [

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

@@ -49,8 +49,10 @@ impl FromIterator<Layer> for RenderLayers {
 
 impl Default for RenderLayers {
     /// By default, this structure includes layer `0`, which represents the first layer.
+    ///
+    /// This is distinct from [`RenderLayers::none`], which doesn't belong to any layers.
     fn default() -> Self {
-        Self::layer(0)
+        const { Self::layer(0) }
     }
 }
 
@@ -68,6 +70,8 @@ impl RenderLayers {
     }
 
     /// Create a new `RenderLayers` that belongs to no layers.
+    ///
+    /// This is distinct from [`RenderLayers::default`], which belongs to the first layer.
     pub const fn none() -> Self {
         RenderLayers(SmallVec::from_const([0; INLINE_BLOCKS]))
     }