modemmanager: improve package README

Explain which are the options supported by the protocol, and also
which are the supported values in each (if the field has value
validation).

Signed-off-by: Aleksander Morgado <aleksander@aleksander.es>

net/modemmanager/README.md

@@ -4,19 +4,33 @@
 
 Cellular modem control and connectivity
 
-Optional libraries libmbim and libqmi are available.  Optional mbim-utils and qmi-utils are available.
-Your modem may require additional kernel modules.
+Optional libraries libmbim and libqmi are available.
+Your modem may require additional kernel modules and/or the usb-modeswitch
+package.
 
 ## Usage
 
-# Once installed, you can configure the 2G/3G/4G modem connections directly in
-   /etc/config/network as in the following example:
+Once installed, you can configure the 2G/3G/4G modem connections directly in
+/etc/config/network as in the following example:
 
     config interface 'broadband'
-        option device   '/sys/devices/platform/soc/20980000.usb/usb1/1-1/1-1.2/1-1.2.1'
-        option proto    'modemmanager'
-        option apn      'ac.vodafone.es'
-        option username 'vodafone'
-        option password 'vodafone'
-        option pincode  '7423'
-        option lowpower '1'
+        option device      '/sys/devices/platform/soc/20980000.usb/usb1/1-1/1-1.2/1-1.2.1'
+        option proto       'modemmanager'
+        option apn         'ac.vodafone.es'
+        option allowedauth 'pap chap'
+        option username    'vodafone'
+        option password    'vodafone'
+        option pincode     '7423'
+        option iptype      'ipv4'
+        option lowpower    '1'
+
+Only 'device' and 'proto' are mandatory options, the remaining ones are all
+optional.
+
+The 'allowedauth' option allows limiting the list of authentication protocols.
+It is given as a space-separated list of values, including any of the
+following: 'pap', 'chap', 'mschap', 'mschapv2' or 'eap'. It will default to
+allowing all protocols.
+
+The 'iptype' option supports any of these values: 'ipv4', 'ipv6' or 'ipv4v6'.
+It will default to 'ipv4' if not given.