Update document to descirbe semantics of hints.

Review URL: http://codereview.chromium.org/1658002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4421 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

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.
    */