[OpenACC][Docs] Add a release note for Clang 21

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
 ^^^^^^^^^^^^^^^^^^^