Add hint to GraalHPyNativeCache about cache layout

Author: fangerer

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/objects/cext/hpy/GraalHPyNativeCache.java

@@ -46,6 +46,9 @@
 
 import sun.misc.Unsafe;
 
+/**
+ * HPy native cache implementation. For documentation of the layout, see {@code hpy_native_cache.h}.
+ */
 public abstract class GraalHPyNativeCache {
 
     private static final Unsafe UNSAFE = CArrayWrappers.UNSAFE;