Help: Mention CUDA Clang limitations in 3.18 release notes

Help/release/3.18.rst

@@ -13,7 +13,8 @@ New Features
 Languages
 ---------
 
-* The ``CUDA`` language can now be compiled using Clang.
+* The ``CUDA`` language can now be compiled using Clang on non-Windows
+  platforms. Separable compilation is not yet supported on any platform.
 
 Command-Line
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