PPP, PPPoS, moved advise about modem from pppos_create() to ppp_open()

pppos_create() can be called whether the modem is ready to process the
PPP session since pppos_create() does not start the PPP session anymore,
moved the advise from pppos_create() to ppp_open().

src/include/netif/ppp/ppp.h

@@ -460,6 +460,9 @@ void ppp_set_notify_phase_callback(ppp_pcb *pcb, ppp_notify_phase_cb_fn notify_p
  *
  * Holdoff is the time to wait (in seconds) before initiating
  * the connection.
+ *
+ * If this port connects to a modem, the modem connection must be
+ * established before calling this.
  */
 err_t ppp_open(ppp_pcb *pcb, u16_t holdoff);