Update system.rs docs in bevy_input (bevyengine#4524)

# Objective

- Part of the splitting process of bevyengine#3692.

## Solution

- Document `system.rs` inside of `bevy_input`.

crates/bevy_input/src/system.rs

@@ -1,11 +1,13 @@
-use crate::{
-    keyboard::{KeyCode, KeyboardInput},
-    ElementState,
-};
+use crate::{ElementState, KeyCode, KeyboardInput};
 use bevy_app::AppExit;
 use bevy_ecs::prelude::{EventReader, EventWriter};
 
-/// Sends the `AppExit` event whenever the "esc" key is pressed.
+/// Sends an [`AppExit`] event whenever the `ESC` key is pressed.
+///
+/// ## Note
+///
+/// This system is not added as part of the `DefaultPlugins`. You can add the [`exit_on_esc_system`]
+/// yourself if desired.
 pub fn exit_on_esc_system(
     mut keyboard_input_events: EventReader<KeyboardInput>,
     mut app_exit_events: EventWriter<AppExit>,