Update document to descirbe semantics of hints.

include/v8.h

Index: include/v8.h
diff --git a/include/v8.h b/include/v8.h
index d90289ab99d27a4ae1f0c614dba73842523b26e7..49417673a2e896ef37b84ed92efd3a94bdc30db8 100644
--- a/include/v8.h
+++ b/include/v8.h
@@ -856,6 +856,8 @@ class V8EXPORT String : public Primitive {
    * copying begins.
    * \param length The number of bytes to copy from the string.
    * \param nchars_ref The number of characters written, can be NULL.
+   * \param hints Various hints that might affect performance of this or
+   *    subsequent operations.
    * \return The number of bytes copied to the buffer
    * excluding the NULL terminator.
    */