[OpenACC][Docs] Add a release note for Clang 21 (#145938)

This patch adds a release note that explains the current status of
OpenACC in Clang. Currently we cannot actually make an executable
because the OpenACC dialect of MLIR doesn't support any amount of
lowering to LLVM-IR, so the usefulness of OpenACC is entirely for
front-end related uses, such as tooling or semantic checking.

clang/docs/ReleaseNotes.rst

@@ -1068,10 +1068,22 @@ Static Analyzer
 New features
 ^^^^^^^^^^^^
 
-A new flag - `-static-libclosure` was introduced to support statically linking
-the runtime for the Blocks extension on Windows. This flag currently only
-changes the code generation, and even then, only on Windows. This does not
-impact the linker behaviour like the other `-static-*` flags.
+- A new flag - `-static-libclosure` was introduced to support statically linking
+  the runtime for the Blocks extension on Windows. This flag currently only
+  changes the code generation, and even then, only on Windows. This does not
+  impact the linker behaviour like the other `-static-*` flags.
+- OpenACC support, enabled via `-fopenacc` has reached a level of completeness
+  to finally be at least notionally usable. Currently, the OpenACC 3.4
+  specification has been completely implemented for Sema and AST creation, so
+  nodes will show up in the AST after having been properly checked. Lowering is
+  currently a work in progress, with compute, loop, and combined constructs
+  partially implemented, plus a handful of data and executable constructs
+  implemented. Lowering will only work in Clang-IR mode (so only with a compiler
+  built with Clang-IR enabled, and with `-fclangir` used on the command line).
+  However, note that the Clang-IR implementation status is also quite partial,
+  so frequent 'not yet implemented' diagnostics should be expected.  Also, the
+  ACC MLIR dialect does not currently implement any lowering to LLVM-IR, so no
+  code generation is possible for OpenACC.
 
 Crash and bug fixes
 ^^^^^^^^^^^^^^^^^^^