Bluetooth: documentation change of kconfig.iso configs

Added comment to KConfig.iso file to make description
of BT_ISO_TX_MTU and BT_ISO_RX_MTU more clear

Signed-off-by: Ivan Iushkov <ivan.iushkov@nordicsemi.no>

subsys/bluetooth/Kconfig.iso

@@ -99,6 +99,10 @@ config BT_ISO_TX_MTU
 	range 1 4095
 	help
 	  Maximum MTU for Isochronous channels TX buffers.
+	  This is the actual data payload. It doesn't include the optional
+	  HCI ISO Data packet fields (e.g. `struct bt_hci_iso_ts_data_hdr`).
+	  Set this value to 247 to fit 247 bytes of data within a single
+	  HCI ISO Data packet with a size of 255, without utilizing timestamps.
 
 config BT_ISO_RX_BUF_COUNT
 	int "Number of Isochronous RX buffers"
@@ -113,6 +117,8 @@ config BT_ISO_RX_MTU
 	range 23 4095
 	help
 	  Maximum MTU for Isochronous channels RX buffers.
+	  This is the actual data payload. It doesn't include the optional
+	  HCI ISO Data packet fields (e.g. `struct bt_hci_iso_ts_data_hdr`)
 
 config BT_ISO_ADVANCED
 	bool "Advanced ISO parameters"