Add premultiplied alpha documentation to StandardMaterial3D tutorial

Author: LunaCapra

tutorials/3d/standard_material_3d.rst

@@ -189,6 +189,9 @@ other than *Mix* forces the object to go through the transparent pipeline.
   screen.
 * **Mul:** The final color of the object is multiplied with the color of the
   screen.
+* **Premultiplied Alpha:** The color of the object is expected to have already been
+  multiplied by the alpha. This behaves like *Add* when the alpha is at zero
+  percent and like *Mix* when the alpha is at one hundred percent.
 
 .. image:: img/spatial_material8.png