Update ClearColor Resource docs (#3842)

# Objective
- Update the `ClearColor` resource docs as described in #3837  so new users (like me) understand it better

## Solution
- Update the docs to use what @alice-i-cecile described in #3837 


I took this one up because I got confused by it this weekend. I didn't understand why the  "background" was being set by a `ClearColor` resource.

crates/bevy_core_pipeline/src/lib.rs

@@ -35,7 +35,10 @@ use bevy_render::{
     RenderApp, RenderStage, RenderWorld,
 };
 
-/// Resource that configures the clear color
+/// When used as a resource, sets the color that is used to clear the screen between frames.
+///
+/// This color appears as the "background" color for simple apps, when
+/// there are portions of the screen with nothing rendered.
 #[derive(Clone, Debug)]
 pub struct ClearColor(pub Color);