Add a TODO in the gcc-plugin part of the doc : the vector_scal example does not work.

There are two problems :

1) size_t arguments do not work in clSetKernelArg
2) "global" should not be set to 1, but to the value of size.

This was "sort of fixed" in the code, but we might want to find a better solution before modifying the documentation.

doc/chapters/basic-examples.texi

@@ -451,6 +451,7 @@ static void vector_scal_opencl (size_t size, float vector[size],
                                 float factor)
   __attribute__ ((task_implementation ("opencl", vector_scal)));
 
+@c TODO This example will not work : size cannot be a size_t in clSetKernelArg, and global should not be 1. Do we want to document the ugly hach we use, though ?
 static void
 vector_scal_opencl (size_t size, float vector[size], float factor)
 @{