Update doc as well.

TBR=ager@chromium.org

Review URL: http://codereview.chromium.org/1619003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4353 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

include/v8.h

@@ -855,7 +855,7 @@ class V8EXPORT String : public Primitive {
    * \param start The starting position within the string at which
    * copying begins.
    * \param length The number of bytes to copy from the string.
-   * \param nchars The number of characters written.
+   * \param nchars_ref The number of characters written, can be NULL.
    * \return The number of bytes copied to the buffer
    * excluding the NULL terminator.
    */