added ftransform() comment

docs/shading.html

@@ -66,6 +66,8 @@ All other major features of the shading language should function.
 <li>Shader error detection and reporting of errors (InfoLog) is not
     very good yet.
 <li>There are known memory leaks in the compiler.
+<li>The ftransform() function doesn't necessarily match the results of
+    fixed-function transformation.
 </ul>
 
 <p>