Add `disable_fog` to Sky shaders documentation (#3906)

tutorials/shading/shading_reference/sky_shader.rst

@@ -96,6 +96,8 @@ a lower resolution than the rest of the sky:
 +---------------------------------+----------------------------------------------------------------------+
 | **use_quarter_res_pass**        | Allows the shader to write to and access the quarter resolution pass.|
 +---------------------------------+----------------------------------------------------------------------+
+| **disable_fog**                 | If used, fog will not affect the sky.                                |
++---------------------------------+----------------------------------------------------------------------+
 
 Built-ins
 ^^^^^^^^^