StandardMaterial docs: Make clear that lighting won't look correct if is_srgb is true (#20037)

# Objective

- Fix #12123

## Solution

- Add a code example found here:
https://github.com/bevyengine/bevy/blob/main/examples/3d/parallax_mapping.rs#L209
to https://docs.rs/bevy/latest/bevy/pbr/struct.StandardMaterial.html

## Testing

- This pull request if only changing docs. But I tested the formatting
via ```cargo doc --no-deps --open```.

---------

Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
Co-authored-by: BD103 <59022059+BD103@users.noreply.github.com>

Author: 3 people

crates/bevy_pbr/src/pbr_material.rs

@@ -386,6 +386,23 @@ pub struct StandardMaterial {
     ///
     /// [`Mesh::generate_tangents`]: bevy_render::mesh::Mesh::generate_tangents
     /// [`Mesh::with_generated_tangents`]: bevy_render::mesh::Mesh::with_generated_tangents
+    ///
+    /// # Usage
+    ///
+    /// ```
+    /// # use bevy_asset::{AssetServer, Handle};
+    /// # use bevy_ecs::change_detection::Res;
+    /// # use bevy_image::{Image, ImageLoaderSettings};
+    /// #
+    /// fn load_normal_map(asset_server: Res<AssetServer>) {
+    ///     let normal_handle: Handle<Image> = asset_server.load_with_settings(
+    ///         "textures/parallax_example/cube_normal.png",
+    ///         // The normal map texture is in linear color space. Lighting won't look correct
+    ///         // if `is_srgb` is `true`, which is the default.
+    ///         |settings: &mut ImageLoaderSettings| settings.is_srgb = false,
+    ///     );
+    /// }
+    /// ```
     #[texture(9)]
     #[sampler(10)]
     #[dependency]