Added note of clip plane being unsupported on OpenGL ES 2.0.

Docs/Reference.dox

@@ -840,6 +840,8 @@ OpenGL ES 2.0 has further limitations:
 
 - For improved performance, shadow mapping quality is reduced: there is no smooth PCF filtering and directional lights can have a maximum of 2 cascades.
 
+- Custom clip planes are not currently supported.
+
 \page Materials Materials
 
 Material and Technique resources define how to render 3D scene geometry. On the disk, they are XML data. By default, materials exist in the Bin/CoreData/Materials & Bin/Data/Materials subdirectories, and techniques exist in the Bin/CoreData/Techniques subdirectory.