Add `CAMERA_VISIBLE_LAYERS` to spatial shader docs (#9244)

(cherry picked from commit 7c8f2cb5efc9c5b9e96bbf00c0e7b8c29aea0f06)

tutorials/shaders/shader_reference/spatial_shader.rst

@@ -169,6 +169,8 @@ shader, this value can be used as desired.
 +----------------------------------------+--------------------------------------------------------+
 | in vec3 **CAMERA_DIRECTION_WORLD**     | Camera world space direction.                          |
 +----------------------------------------+--------------------------------------------------------+
+| in int **CAMERA_VISIBLE_LAYERS**       | Cull layers of the camera rendering the current pass.  |
++----------------------------------------+--------------------------------------------------------+
 | in bool **OUTPUT_IS_SRGB**             | ``true`` when output is in sRGB color space            |
 |                                        | (this is ``true`` in the Compatibility renderer,       |
 |                                        | ``false`` in Forward+ and Forward Mobile).             |