added hint for use of RPromise.resetValue()

com.oracle.truffle.r.runtime/src/com/oracle/truffle/r/runtime/data/RPromise.java

@@ -204,7 +204,8 @@ public class RPromise implements RTypedValue {
     }
 
     /**
-     * Discards any previously evaluated value if this is not an eager promise.
+     * Discards any previously evaluated value if this is not an eager promise. This is for
+     * debugging purposes only!
      */
     public final void resetValue() {
         // Only non-eager promises can be reset.