[OpenCL][Doc] Description for adding OpenCL vendor extension in user manual

[Anastasia]

Summary

Adding a description of a new feature that allows to specify vendor extension in flexible way using compiler pragma instead of modifying source code directly.

Feature committed in Clang@r289979.

[yaxunl]

LGTM. Thanks!

[Anastasia]

Committed in 295313!

docs/UsersManual.rst

can be given manually.
 
    .. code-block:: console
 
      $ clang -I<path to clang>/lib/Headers/opencl-c.h test.cl
 
 In this case the kernel code should contain ``#include <opencl-c.h>`` just as a
 regular C include.
 
+.. _opencl_cl_ext:
+
 .. option:: -cl-ext
 
 Disables support of OpenCL extensions. All OpenCL targets provide a list
 of extensions that they support. Clang allows to amend this using the ``-cl-ext``
 flag with a comma-separated list of extensions prefixed with ``'+'`` or ``'-'``.
 The syntax: ``-cl-ext=<(['-'|'+']<extension>[,])+>``,  where extensions
 can be either one of `the OpenCL specification extensions
 <https://www.khronos.org/registry/cl/sdk/2.0/docs/man/xhtml/EXTENSION.html>`_

 speed.
 
 To enable modules for OpenCL:
 
    .. code-block:: console
 
      $ clang -target spir-unknown-unknown -c -emit-llvm -Xclang -finclude-default-header -fmodules -fimplicit-module-maps -fmodules-cache-path=<path to the generated module> test.cl
 
+OpenCL Extensions
+-----------------
+
+All of the ``cl_khr_*`` extensions from `the official OpenCL specification
+<https://www.khronos.org/registry/OpenCL/sdk/2.0/docs/man/xhtml/EXTENSION.html>`_
+up to and including version 2.0 are available and set per target depending on the
+support available in the specific architecture.
+
+It is possible to alter the default extensions setting per target using
+``-cl-ext`` flag. (See :ref:`flags description <opencl_cl_ext>` for more details).
+
+Vendor extensions can be added flexibly by declaring the list of types and
+functions associated with each extensions enclosed within the following
+compiler pragma directives:
+
+  .. code-block:: c
+
+       #pragma OPENCL EXTENSION the_new_extension_name : begin
+       // declare types and functions associated with the extension here
+       #pragma OPENCL EXTENSION the_new_extension_name : end
+
+For example, parsing the following code adds ``my_t`` type and ``my_func``
+function to the custom ``my_ext`` extension.
+
+  .. code-block:: c
+
+       #pragma OPENCL EXTENSION my_ext : begin
+       typedef struct{
+         int a;
+       }my_t;
+       void my_func(my_t);
+       #pragma OPENCL EXTENSION my_ext : end
+
+Declaring the same types in different vendor extensions is disallowed.
+
 OpenCL Metadata
 ---------------
 
 Clang uses metadata to provide additional OpenCL semantics in IR needed for
 backends and OpenCL runtime.
 
 Each kernel will have function metadata attached to it, specifying the arguments.
 Kernel argument metadata is used to provide source level information for querying

 ^^^^^
 
 Clang supports this attribute to comply to OpenCL v2.0 conformance, but it
 does not have any effect on the IR. For more details reffer to the specification
 `section 6.7.2
 <https://www.khronos.org/registry/cl/specs/opencl-2.0-openclc.pdf#49>`_
 
 
-opencl_hint_unroll
+opencl_unroll_hint
 ^^^^^^^^^^^^^^^^^^
 
 The implementation of this feature mirrors the unroll hint for C.
 More details on the syntax can be found in the specification
 `section 6.11.5
 <https://www.khronos.org/registry/cl/specs/opencl-2.0-openclc.pdf#61>`_
 
 convergent