[docs] Mention another fixed bug in the release notes

Author: mstorsjo

llvm/docs/ReleaseNotes.rst

@@ -147,6 +147,10 @@ Changes to the Windows Target
   constructors), making sure that GNU ld exports all of them correctly as
   functions, not data, when linking a DLL.
 
+* Handling of temporary files on more uncommon file systems (network
+  mounts, ramdisks) on Windows is fixed now (which previously either
+  errored out or left stray files behind).
+
 Changes to the OCaml bindings
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