[ReleaseNotes] Mention the `vector-function-abi-variant` attribute.

[kristof.beyls]

As someone who isn't up to speed on the design of vector-function-abi-variant, this release note makes sense to me.
It'd probably be best though for this to be approved by someone who understands the design.

[fpetrogalli]

Adding @jdoerfert and @andwar who has been involved in the technicalities related to this attribute.

[andwar]

LGTM

llvm/docs/ReleaseNotes.rst

.. NOTE
    -------------------
 
    Makes programs 10x faster by doing Special New Thing.
 
 
 Changes to the LLVM IR
 ----------------------
 
+* The callsite attribute `vector-function-abi-variant
+  <https://llvm.org/docs/LangRef.html#call-site-attributes>`_ has been
+  added to describe the mapping between scalar functions and vector
+  functions, to enable vectorization of call sites. The information
+  provided by the attribute is interfaced via the API provided by the
+  ``VFDatabase`` class.
 
 Changes to building LLVM
 ------------------------
 
 Changes to the ARM Backend
 --------------------------
 
 During this release ...