*
* Returns nonzero on success. You cannot register twice for the same
* extension number, and registering for an extension number already
- * handled by OpenSSL will succeed, but the callbacks will not be invoked.
+ * handled by OpenSSL will fail.
*
* NULL can be registered for any callback function. For the client
* functions, a NULL custom_cli_ext_first_cb_fn sends an empty ClientHello