Add comment about unicorn_Unicorn.h to the .c file

bindings/java/unicorn_Unicorn.c

@@ -19,6 +19,9 @@ Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
 
 */
 
+/** Note: JNI function signatures and names must be kept in sync with
+    unicorn_Unicorn.h, which is in turn auto-generated by `javac -h`. */
+
 #include <sys/types.h>
 #include "unicorn/platform.h"
 #include <stdlib.h>