Add documentation to VisibleEntities and related (bevyengine#5100)

# Objective

Add missing docs

## Solution

Add documentation to the `VisibleEntities` component, its related
`check_visibility()` system, and that system's label.

See Discord discussion here : https://discord.com/channels/691052431525675048/866787577687310356/990432663921901678

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

@@ -49,6 +49,18 @@ impl Default for ComputedVisibility {
 #[derive(Component)]
 pub struct NoFrustumCulling;
 
+/// Collection of entities visible from the current view.
+///
+/// This component contains all entities which are visible from the currently
+/// rendered view. The collection is updated automatically by the [`check_visibility()`]
+/// system, and renderers can use it to optimize rendering of a particular view, to
+/// prevent drawing items not visible from that view.
+///
+/// This component is intended to be attached to the same entity as the [`Camera`] and
+/// the [`Frustum`] defining the view.
+///
+/// Currently this component is ignored by the sprite renderer, so sprite rendering
+/// is not optimized per view.
 #[derive(Clone, Component, Default, Debug, Reflect)]
 #[reflect(Component)]
 pub struct VisibleEntities {
@@ -76,6 +88,8 @@ pub enum VisibilitySystems {
     UpdateOrthographicFrusta,
     UpdatePerspectiveFrusta,
     UpdateProjectionFrusta,
+    /// Label for the [`check_visibility()`] system updating each frame the [`ComputedVisibility`]
+    /// of each entity and the [`VisibleEntities`] of each view.
     CheckVisibility,
 }
 
@@ -149,6 +163,11 @@ pub fn update_frusta<T: Component + CameraProjection + Send + Sync + 'static>(
     }
 }
 
+/// System updating the visibility of entities each frame.
+///
+/// The system is labelled with [`VisibilitySystems::CheckVisibility`]. Each frame, it updates the
+/// [`ComputedVisibility`] of all entities, and for each view also compute the [`VisibleEntities`]
+/// for that view.
 pub fn check_visibility(
     mut thread_queues: Local<ThreadLocal<Cell<Vec<Entity>>>>,
     mut view_query: Query<(&mut VisibleEntities, &Frustum, Option<&RenderLayers>), With<Camera>>,