[C++ for OpenCL] Elaborate behavior of extensions (#740)

Extensions from OpenCL C are also valid in C++ for OpenCL.

Author: AnastasiaStulova

cxx4opencl/intro.txt

@@ -44,3 +44,6 @@ not covered in either OpenCL or {cpp} specifications, in particular:
  * any behavior that deviates from {cpp}17;
  * any behavior that deviates from OpenCL C v2.0;
  * any behavior that is not governed by OpenCL C and {cpp}.
+
+All language extensions to OpenCL C v2.0 or earlier are applicable to
+C++ for OpenCL.