add default direction to DirectionalLight docs (bevyengine#5188)

# Objective

- Adds a default direction to the documentation of DirectionalLight 

## Solution

Suggestion from Q&A answer: 
bevyengine#5186 (reply in thread)

crates/bevy_pbr/src/light.rs

@@ -144,6 +144,9 @@ impl Default for SpotLight {
 /// approximation for light sources VERY far away, like the sun or
 /// the moon.
 ///
+/// The light shines along the forward direction of the entity's transform. With a default transform
+/// this would be along the negative-Z axis.
+///
 /// Valid values for `illuminance` are:
 ///
 /// | Illuminance (lux) | Surfaces illuminated by                        |