[OpenCL][Docs] Minor update about C++ for OpenCL in UsersManual.

Author: Anastasia Stulova

clang/docs/UsersManual.rst

@@ -3237,6 +3237,18 @@ compiling ``.cl`` file ``-cl-std=clc++``, ``-cl-std=CLC++``, ``-std=clc++`` or
 
      clang -cl-std=clc++ test.cl
 
+Alternatively, files with ``.clcpp`` extension are compiled with the C++ for OpenCL
+mode.
+
+   .. code-block:: console
+
+     clang test.clcpp
+
+C++ for OpenCL kernel sources can also be compiled online in drivers supporting 
+`cl_ext_cxx_for_opencl
+<https://www.khronos.org/registry/OpenCL/extensions/ext/cl_ext_cxx_for_opencl.html>`_
+extension.
+
 Constructing and destroying global objects
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^