[ReleaseNotes] Mention new matrix intrinsics.

Reviewers: anemet, Gerolf

Reviewed By: anemet

Differential Revision: https://reviews.llvm.org/D75161

Author: fhahn

llvm/docs/LangRef.rst

@@ -14553,6 +14553,9 @@ Arguments:
 """"""""""
 The argument to this intrinsic must be a vector of floating-point values.
 
+
+.. _i_matrixintrinsics:
+
 Matrix Intrinsics
 -----------------
 

llvm/docs/ReleaseNotes.rst

@@ -106,6 +106,12 @@ Non-comprehensive list of changes in this release
   still under heavy development and disabled by default, to enable an early run
   pass ``-mllvm -attributor-disable=false`` to an invocation of clang.
 
+* New matrix math intrinsics have been added to LLVM
+  (see :ref:`LLVM Language Reference Manual <i_matrixintrinsics>`), together
+  with the LowerMatrixIntrinsics pass. The pass lowers matrix intrinsics
+  to a set of efficient vector instructions. The lowering pass is off
+  by default and can be enabled by passing ``-mllvm -enable-matrix`` to an
+  invocation of clang.
 
 
 Changes to the LLVM IR