ipconfig: Document /proc/net/pnp

Fully document the format used by the /proc/net/pnp file written by
ipconfig, explain where its values originate from, and clarify that the
tertiary name server IP and DNS domain name are only written to the file
when autoconfiguration is used.
Signed-off-by: Chris Novakovic <chris@chrisn.me.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/filesystems/nfs/nfsroot.txt

@@ -110,6 +110,9 @@ ip=<client-ip>:<server-ip>:<gw-ip>:<netmask>:<hostname>:<device>:<autoconf>:
 		will not be triggered if it is missing and NFS root is not
 		in operation.
 
+		Value is exported to /proc/net/pnp with the prefix "bootserver "
+		(see below).
+
 		Default: Determined using autoconfiguration.
 		         The address of the autoconfiguration server is used.
 
@@ -165,12 +168,33 @@ ip=<client-ip>:<server-ip>:<gw-ip>:<netmask>:<hostname>:<device>:<autoconf>:
 
                 Default: any
 
-  <dns0-ip>	IP address of first nameserver.
-		Value gets exported by /proc/net/pnp which is often linked
-		on embedded systems by /etc/resolv.conf.
+  <dns0-ip>	IP address of primary nameserver.
+		Value is exported to /proc/net/pnp with the prefix "nameserver "
+		(see below).
+
+		Default: None if not using autoconfiguration; determined
+		automatically if using autoconfiguration.
+
+  <dns1-ip>	IP address of secondary nameserver.
+		See <dns0-ip>.
+
+  After configuration (whether manual or automatic) is complete, a file is
+  created at /proc/net/pnp in the following format; lines are omitted if
+  their respective value is empty following configuration.
+
+	#PROTO: <DHCP|BOOTP|RARP|MANUAL>	(depending on configuration method)
+	domain <dns-domain>			(if autoconfigured, the DNS domain)
+	nameserver <dns0-ip>			(primary name server IP)
+	nameserver <dns1-ip>			(secondary name server IP)
+	nameserver <dns2-ip>			(tertiary name server IP)
+	bootserver <server-ip>			(NFS server IP)
+
+  <dns-domain> and <dns2-ip> are requested during autoconfiguration; they
+  cannot be specified as part of the "ip=" kernel command line parameter.
 
-  <dns1-ip>	IP address of second nameserver.
-		Same as above.
+  Because the "domain" and "nameserver" options are recognised by DNS
+  resolvers, /etc/resolv.conf is often linked to /proc/net/pnp on systems
+  that use an NFS root filesystem.
 
 
 nfsrootdebug