Re-introduce comments about frustum culling (bevyengine#8579)

# Objective

Frustum culling for 2D components has been enabled since bevyengine#7885,
Fixes bevyengine#8490 

## Solution

Re-introduced the comments about frustum culling in the
many_animated_sprites.rs and many_sprites.rs examples.

---------

Co-authored-by: Nicola Papale <nicopap@users.noreply.github.com>
Co-authored-by: François <mockersf@gmail.com>

examples/stress_tests/many_animated_sprites.rs

@@ -1,10 +1,7 @@
 //! Renders a lot of animated sprites to allow performance testing.
 //!
-//! It sets up many animated sprites in different sizes and rotations,
-//! and at different scales in the world, and moves the camera over them.
-//!
-//! Having sprites out of the camera's field of view should also help stress
-//! test any future potential 2d frustum culling implementation.
+//! This example sets up many animated sprites in different sizes, rotations, and scales in the world.
+//! It also moves the camera over them to see how well frustum culling works.
 
 use std::time::Duration;
 

examples/stress_tests/many_sprites.rs

@@ -1,11 +1,8 @@
 //! Renders a lot of sprites to allow performance testing.
 //! See <https://github.com/bevyengine/bevy/pull/1492>
 //!
-//! It sets up many animated sprites in different sizes and rotations,
-//! and at different scales in the world, and moves the camera over them.
-//!
-//! Having sprites out of the camera's field of view should also help stress
-//! test any future potential 2d frustum culling implementation.
+//! This example sets up many sprites in different sizes, rotations, and scales in the world.
+//! It also moves the camera over them to see how well frustum culling works.
 //!
 //! Add the `--colored` arg to run with color tinted sprites. This will cause the sprites to be rendered
 //! in multiple batches, reducing performance but useful for testing.