Explain how AmbientLight is inserted and configured (bevyengine#10712)

crates/bevy_pbr/src/light.rs

@@ -544,6 +544,20 @@ fn calculate_cascade(
 }
 
 /// An ambient light, which lights the entire scene equally.
+///
+/// This resource is inserted by the [`PbrPlugin`] and by default it is set to a low ambient light.
+///
+/// # Examples
+///
+/// Make ambient light slightly brighter:
+///
+/// ```
+/// # use bevy_ecs::system::ResMut;
+/// # use bevy_pbr::AmbientLight;
+/// fn setup_ambient_light(mut ambient_light: ResMut<AmbientLight>) {
+///    ambient_light.brightness = 0.3;
+/// }
+/// ```
 #[derive(Resource, Clone, Debug, ExtractResource, Reflect)]
 #[reflect(Resource)]
 pub struct AmbientLight {