Add description for custom vertex values

Co-Authored-By: Clay John <[email protected]>

tutorials/shaders/shader_reference/canvas_item_shader.rst

@@ -132,9 +132,9 @@ is usually:
 +--------------------------------+----------------------------------------------------+
 | inout float **POINT_SIZE**     | Point size for point drawing.                      |
 +--------------------------------+----------------------------------------------------+
-| in vec4 **CUSTOM0**            |                                                    |
+| in vec4 **CUSTOM0**            | Custom value from vertex primitive.                |
 +--------------------------------+----------------------------------------------------+
-| in vec4 **CUSTOM1**            |                                                    |
+| in vec4 **CUSTOM1**            | Custom value from vertex primitive.                |
 +--------------------------------+----------------------------------------------------+
 
 Fragment built-ins