Added documentation explaining the difference between lumens and luxes (

bevyengine#11551)

# Objective

- Fix bevyengine#9809

## Solution

- Describe the difference between lumens and luxes, and why some
lightsources use one and some the other.

crates/bevy_pbr/src/light.rs

@@ -41,7 +41,7 @@ use crate::*;
 #[reflect(Component, Default)]
 pub struct PointLight {
     pub color: Color,
-    /// Luminous power in lumens
+    /// Luminous power in lumens, representing the amount of light emitted by this source in all directions.
     pub intensity: f32,
     pub range: f32,
     pub radius: f32,
@@ -92,7 +92,7 @@ impl Default for PointLightShadowMap {
 #[reflect(Component, Default)]
 pub struct SpotLight {
     pub color: Color,
-    /// Luminous power in lumens
+    /// Luminous power in lumens, representing the amount of light emitted by this source in all directions.
     pub intensity: f32,
     pub range: f32,
     pub radius: f32,
@@ -188,7 +188,12 @@ impl Default for SpotLight {
 #[reflect(Component, Default)]
 pub struct DirectionalLight {
     pub color: Color,
-    /// Illuminance in lux
+    /// Illuminance in lux (lumens per square meter), representing the amount of
+    /// light projected onto surfaces by this light source. Lux is used here
+    /// instead of lumens because a directional light illuminates all surfaces
+    /// more-or-less the same way (depending on the angle of incidence). Lumens
+    /// can only be specified for light sources which emit light from a specific
+    /// area.
     pub illuminance: f32,
     pub shadows_enabled: bool,
     pub shadow_depth_bias: f32,