Document ClearColorConfig (#9288)

# Objective

Document the `ClearColorConfig` enum, and describe the behavior of its
variants.

crates/bevy_core_pipeline/src/clear_color.rs

@@ -4,12 +4,18 @@ use bevy_reflect::{Reflect, ReflectDeserialize, ReflectSerialize};
 use bevy_render::{color::Color, extract_resource::ExtractResource};
 use serde::{Deserialize, Serialize};
 
+/// For a camera, specifies the color used to clear the viewport before rendering.
 #[derive(Reflect, Serialize, Deserialize, Clone, Debug, Default)]
 #[reflect(Serialize, Deserialize)]
 pub enum ClearColorConfig {
+    /// The clear color is taken from the world's [`ClearColor`] resource.
     #[default]
     Default,
+    /// The given clear color is used, overriding the [`ClearColor`] resource defined in the world.
     Custom(Color),
+    /// No clear color is used: the camera will simply draw on top of anything already in the viewport.
+    ///
+    /// This can be useful when multiple cameras are rendering to the same viewport.
     None,
 }