[DTLTO] Add COFF LLD release note for LLVM 21

DTLTO-related COFF LLD changes were cherry-picked to the LLVM 21
release branch in:
  #149979

This commit adds the corresponding release note, modeled after
the previously added note for ELF LLD DTLTO support.

Author: bd1976bris

lld/docs/ReleaseNotes.rst

@@ -92,6 +92,9 @@ Breaking changes
 
 COFF Improvements
 -----------------
+* ``/thinlto-distributor`` and ``/thinlto-remote-compiler`` options
+  added to support Integrated Distributed ThinLTO.
+  (`#147265 <https://github.com/llvm/llvm-project/pull/147265>`_)
 
 MinGW Improvements
 ------------------