added documentation for position built-in

tutorials/shading/shading_reference/spatial_shader.rst

@@ -81,7 +81,7 @@ as they came.
 
 They can optionally be presented in world space by using the *world_vertex_coords* render mode.
 
-User can disable the built-in modelview transform (projection will still happen later) and do 
+Users can disable the built-in modelview transform (projection will still happen later) and do 
 it manually with the following code:
 
 .. code-block:: glsl
@@ -90,7 +90,6 @@ it manually with the following code:
     render_mode skip_vertex_transform;
 
     void vertex() {
-
         VERTEX = (MODELVIEW_MATRIX * vec4(VERTEX, 1.0)).xyz;
         NORMAL = (MODELVIEW_MATRIX * vec4(NORMAL, 0.0)).xyz;
         // same as above for binormal and tangent, if normal mapping is used
@@ -98,6 +97,10 @@ it manually with the following code:
 
 Other built-ins such as UV, UV2 and COLOR are also passed through to the fragment function if not modified.
 
+Users can override the modelview and projection transforms using the ``POSITION`` built-in. When ``POSITION`` is used
+the value from ``VERTEX`` is ignored and projection does not happen. However, the value passed to the fragment shader 
+still comes from ``VERTEX``.
+
 For instancing, the INSTANCE_CUSTOM variable contains the instance custom data. When using particles, this information
 is usually:
 
@@ -143,6 +146,8 @@ shader, this value can be used as desired.
 +------------------------------------+-------------------------------------------------------+
 | out float **POINT_SIZE**           | Point size for point rendering.                       |
 +------------------------------------+-------------------------------------------------------+
+| out vec4  **POSITION**             | If written to, overrides final vertex position.       |
++------------------------------------+-------------------------------------------------------+
 | in int **INSTANCE_ID**             | Instance ID for instancing.                           |
 +------------------------------------+-------------------------------------------------------+
 | in vec4 **INSTANCE_CUSTOM**        | Instance custom data (for particles, mostly).         |