Docs for emissive color on StandardMaterial (#20497)

# Objective

Fixes #18993.

## Solution

I've done my best to synthesize and carefully word the discussion in the
linked issue. That said, this is not my domain, so critique is very
welcome.

Author: alice-i-cecile

crates/bevy_pbr/src/pbr_material.rs

@@ -78,16 +78,25 @@ pub struct StandardMaterial {
     ///
     /// The default emissive color is [`LinearRgba::BLACK`], which doesn't add anything to the material color.
     ///
-    /// To increase emissive strength, channel values for `emissive`
+    /// Emissive strength is controlled by the value of the color channels,
+    /// while the hue is controlled by their relative values.
+    ///
+    /// As a result, channel values for `emissive`
     /// colors can exceed `1.0`. For instance, a `base_color` of
     /// `LinearRgba::rgb(1.0, 0.0, 0.0)` represents the brightest
     /// red for objects that reflect light, but an emissive color
     /// like `LinearRgba::rgb(1000.0, 0.0, 0.0)` can be used to create
     /// intensely bright red emissive effects.
     ///
+    /// This results in a final luminance value when multiplied
+    /// by the value of the greyscale emissive texture (which ranges from 0 for black to 1 for white).
+    /// Luminance is a measure of the amount of light emitted per unit area,
+    /// and can be thought of as the "brightness" of the effect.
+    /// In Bevy, we treat these luminance values as the physical units of cd/m², aka nits.
+    ///
     /// 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**,
+    /// typically light up surrounding areas like a light source**,
     /// it just adds a value to the color seen on screen.
     pub emissive: LinearRgba,