[NFC][AArch64] Add relnote saying modal FP8 intrinsics now fully implemented by ACLE (#141743)

Author: jthackray

clang/docs/ReleaseNotes.rst

@@ -851,6 +851,10 @@ X86 Support
 Arm and AArch64 Support
 ^^^^^^^^^^^^^^^^^^^^^^^
 
+- Implementation of modal 8-bit floating point intrinsics in accordance with
+  the Arm C Language Extensions (ACLE)
+  `as specified here <https://github.com/ARM-software/acle/blob/main/main/acle.md#modal-8-bit-floating-point-extensions>`_
+  is now available.
 - Support has been added for the following processors (command-line identifiers in parentheses):
   - Arm Cortex-A320 (``cortex-a320``)
 - For ARM targets, cc1as now considers the FPU's features for the selected CPU or Architecture.