[Clang][Doc][OpenCL] Release 16 notes

Documented major OpenCL features in release 16.

Differential Revision: https://reviews.llvm.org/D143348

Author: AnastasiaStulova

clang/docs/ReleaseNotes.rst

@@ -395,6 +395,18 @@ C2x Feature Support
   Also documents the builtin appropriately. Note, a type definition in C++
   continues to be rejected.
 
+OpenCL Kernel Language Changes
+------------------------------
+
+- Improved diagnostics for C++ templates used in kernel arguments.
+- Removed redundant pragma for the ``arm-integer-dot-product`` extension.
+- Improved support of enqueued block for AMDGPU.
+- Added ``nounwind`` attribute implicitly to all functions.
+- Improved builtin functions support:
+
+  * Allow disabling default header-based feature/extension support by passing ``-D__undef_<feature>``.
+  * Fixed conditional definition of the depth image and ``read_write image3d`` builtins.
+
 Non-comprehensive list of changes in this release
 -------------------------------------------------
 - It's now possible to set the crash diagnostics directory through