nixos/doc/rl-2505: add highlight for the LLVM bump

nixos/doc/manual/release-notes/rl-2505.section.md

@@ -13,6 +13,9 @@
 - GCC has been updated from GCC 13 to GCC 14.
   This introduces some backwards‐incompatible changes; see the [upstream porting guide](https://gcc.gnu.org/gcc-14/porting_to.html) for details.
 
+- LLVM has been updated from LLVM 16 (on Darwin) and LLVM 18 (on other platforms) to LLVM 19.
+  This introduces some backwards‐incompatible changes; see the [upstream release notes](https://releases.llvm.org/) for details.
+
 <!-- To avoid merge conflicts, consider adding your item at an arbitrary place in the list instead. -->
 
 ## New Modules {#sec-release-25.05-new-modules}