[lld][macho][NFC] Add release note for llvm#158720 (llvm#161295)

I forgot to add a release note for
llvm#158720 so I'll add it here.

Author: ellishg

lld/docs/ReleaseNotes.rst

@@ -44,6 +44,9 @@ MinGW Improvements
 MachO Improvements
 ------------------
 
+* ``--separate-cstring-literal-sections`` emits cstring literal sections into sections defined by their section name.
+  (`#158720 <https://github.com/llvm/llvm-project/pull/158720>`_)
+
 WebAssembly Improvements
 ------------------------