drivers: modem: bg9x: Update help description

Add note about PSM/eDRX modes to make it clear that off-load driver
still important when using in baterry powered devices.

Signed-off-by: BUDKE Gerson Fernando <gerson.budke@leica-geosystems.com>

Author: nandojve

drivers/modem/Kconfig.quectel-bg9x

@@ -11,8 +11,10 @@ config MODEM_QUECTEL_BG9X
 	select MODEM_SOCKET
 	select NET_SOCKETS_OFFLOAD
 	help
-	  Choose this setting to enable quectel BG9x LTE-CatM1/NB-IoT modem
-	  driver.
+	  Quectel BG9x LTE-CatM1/NB-IoT off-load modem driver. This should be
+	  used when off-load drivers are needed. This is the preferred mode
+	  only when user needs PSM/eDRX power saving modes. Otherwise, the
+	  celullar modem is the recommended solution.
 
 if MODEM_QUECTEL_BG9X