[OpenMP][Docs] Update OpenMP supported features table (#115106)

OpenMP features table: added 'allocator' modifier for 'allocate' clause
as a completed feature in OpenMP 5.1 Implementation Details.

Author: ddpagan

clang/docs/OpenMPSupport.rst

@@ -284,6 +284,8 @@ implementation.
 +------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
 | memory management            | alignment for allocate directive and clause                  | :good:`done`             | D115683                                                               |
 +------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
+| memory management            | 'allocator' modifier for allocate clause                     | :good:`done`             | https://github.com/llvm/llvm-project/pull/114883                      |
++------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
 | memory management            | new memory management routines                               | :none:`unclaimed`        |                                                                       |
 +------------------------------+--------------------------------------------------------------+--------------------------+-----------------------------------------------------------------------+
 | memory management            | changes to omp_alloctrait_key enum                           | :none:`unclaimed`        |                                                                       |