[OpenCL][Docs] Improved description of the supported language functionality.

[Anastasia]

Summary

As suggested on RFC: http://lists.llvm.org/pipermail/cfe-dev/2020-October/066932.html

[Anastasia]

clang/docs/UsersManual.rst

2718

I am surprised this doesn't appear on the registry page though. It would be nicer to link a pdf or an html instead of sources.

[azabaznov]

Looks good to me, thanks for the note.

clang/docs/UsersManual.rst

 
 Clang can be used to compile OpenCL kernels for execution on a device
 (e.g. GPU). It is possible to compile the kernel into a binary (e.g. for AMD or
 Nvidia targets) that can be uploaded to run directly on a device (e.g. using
 `clCreateProgramWithBinary
 <https://www.khronos.org/registry/OpenCL/specs/opencl-1.1.pdf#111>`_) or
 into generic bitcode files loadable into other toolchains.
 
+Clang only supports the full OpenCL profile, and not `the embedded profile
+<https://github.com/KhronosGroup/OpenCL-Docs/blob/master/api/embedded_profile.asciidoc>`_.
+
 Compiling to a binary using the default target from the installation can be done
 as follows:
 
    .. code-block:: console
 
      $ echo "kernel void k(){}" > test.cl
      $ clang test.cl