Specify units in AmbientLight::brightness docs (bevyengine#13297)

# Objective

- Fixes bevyengine#11933.
- Related: bevyengine#12280.

## Solution

- Specify that, after applying `AmbientLight`, the resulting units are
in cd/m^2.
- This is based on [@fintelia's
comment](bevyengine#11933 (comment)),
and will need to be verified.

---

## Changelog

- Specified units for `AmbientLight`'s `brightness` field.

crates/bevy_pbr/src/light/ambient_light.rs

@@ -20,6 +20,10 @@ use super::*;
 pub struct AmbientLight {
  pub color: Color,
  /// A direct scale factor multiplied with `color` before being passed to the shader.
+ ///
+ /// After applying this multiplier, the resulting value should be in units of [cd/m^2].
+ ///
+ /// [cd/m^2]: https://en.wikipedia.org/wiki/Candela_per_square_metre
  pub brightness: f32,
 }