update comment on emissive field of StandardMaterial struct to mention large color channel values (#12248)

# Objective

- Describe the objective or issue this PR addresses.

Improve docs around emissive colors --

I couldn't figure out how to increase the emissive strength of
materials, asking on discord @alice-i-cecile told me that color channel
values can go above `1.0` in the case of the `emissive` field. I would
have never figured this out on my own, because [the docs for
emissive](https://docs.rs/bevy/latest/bevy/prelude/struct.StandardMaterial.html#structfield.emissive)
don't mention this possibility, and indeed if you follow the link in the
`emissive` doc [to the `Color`
type](https://docs.rs/bevy/latest/bevy/render/color/enum.Color.html#variants),
you are told that values should be in `[0.0, 1.0]`.


## Solution

- Describe the solution used to achieve the objective above.

Just added a note on the possibility of large color channel values with
example.

crates/bevy_pbr/src/pbr_material.rs

@@ -59,7 +59,16 @@ pub struct StandardMaterial {
     ///
     /// The default emissive color is [`Color::BLACK`], which doesn't add anything to the material color.
     ///
-    /// Note that **an emissive material won't light up surrounding areas like a light source**,
+    /// To increase emissive strength, channel values for `emissive`
+    /// colors can exceed `1.0`. For instance, a `base_color` of
+    /// `Color::linear_rgb(1.0, 0.0, 0.0)` represents the brightest
+    /// red for objects that reflect light, but an emissive color
+    /// like `Color::linear_rgb(1000.0, 0.0, 0.0)` can be used to create
+    /// intensely bright red emissive effects.
+    ///
+    /// Increasing the emissive strength of the color will impact visual effects
+    /// like bloom, but it's important to note that **an emissive material won't
+    /// light up surrounding areas like a light source**,
     /// it just adds a value to the color seen on screen.
     pub emissive: Color,