Set ambient_intensity to 0.0 in volumetric_fog example, correct doc comment (#13531)

# Objective

- Fixes #13521

## Solution

Set `ambient_intensity` to 0.0 in volumetric_fog example.

I chose setting it explicitly over changing the default in order to make
it clear that this needs to be set depending on whether you have an
`EnvironmentMapLight`. See documentation for `ambient_intensity` and
related members.

## Testing

- Run the volumetric_fog example and notice how the light shown in
#13521 is not there anymore, as expected.

---------

Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>

crates/bevy_pbr/src/volumetric_fog/mod.rs

@@ -103,18 +103,19 @@ pub struct VolumetricFogSettings {
 
     /// Color of the ambient light.
     ///
-    /// This is separate from Bevy's [`crate::light::AmbientLight`] because an
-    /// [`EnvironmentMapLight`] is still considered an ambient light for the
-    /// purposes of volumetric fog. If you're using a
-    /// [`crate::EnvironmentMapLight`], for best results, this should be a good
-    /// approximation of the average color of the environment map.
+    /// This is separate from Bevy's [`AmbientLight`](crate::light::AmbientLight) because an
+    /// [`EnvironmentMapLight`](crate::environment_map::EnvironmentMapLight) is
+    /// still considered an ambient light for the purposes of volumetric fog. If you're using a
+    /// [`EnvironmentMapLight`](crate::environment_map::EnvironmentMapLight), for best results,
+    /// this should be a good approximation of the average color of the environment map.
     ///
     /// Defaults to white.
     pub ambient_color: Color,
 
     /// The brightness of the ambient light.
     ///
-    /// If there's no ambient light, set this to 0.
+    /// If there's no [`EnvironmentMapLight`](crate::environment_map::EnvironmentMapLight),
+    /// set this to 0.
     ///
     /// Defaults to 0.1.
     pub ambient_intensity: f32,

examples/3d/volumetric_fog.rs

@@ -50,7 +50,11 @@ fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
             image: asset_server.load("environment_maps/pisa_specular_rgb9e5_zstd.ktx2"),
             brightness: 1000.0,
         })
-        .insert(VolumetricFogSettings::default());
+        .insert(VolumetricFogSettings {
+            // This value is explicitly set to 0 since we have no environment map light
+            ambient_intensity: 0.0,
+            ..default()
+        });
 
     // Add the help text.
     commands.spawn(