[clang] [OpenMP] Add docs for fb_nullify/fb_preserve (PR #169558)

[Abhinav Gaba via cfe-commits]

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@@ -580,6 +580,8 @@ implementation.
 | need_device_addr modifier for adjust_args clause            | :part:`partial`           | :none:`unclaimed`         | Parsing/Sema: https://github.com/llvm/llvm-project/pull/143442           |
 |                                                             |                           |                           |               https://github.com/llvm/llvm-project/pull/149586           |
 +-------------------------------------------------------------+---------------------------+---------------------------+--------------------------------------------------------------------------+
+| need_device_ptr modifier for adjust_args clause             | :part:`partial`           | :none:`unclaimed`         | Clang Parsing/Sema: https://github.com/llvm/llvm-project/pull/168905     |
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abhinavgaba wrote:

It makes sense to have a row for `need_device_ptr` in the OpenMP 6.0 section, but there should also be one for the optional fallback argument in the OpenMP 6.1 section, and the PR link should go in that 6.1 row.

https://github.com/llvm/llvm-project/pull/169558