ipconfig: document DHCP hostname and DNS record

Now it's possible to update the DNS record for $HOST_NAME with

	ip=::::$HOST_NAME::dhcp

CC: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/filesystems/nfs/nfsroot.txt

@@ -124,6 +124,8 @@ ip=<client-ip>:<server-ip>:<gw-ip>:<netmask>:<hostname>:<device>:<autoconf>
 
   <hostname>	Name of the client. May be supplied by autoconfiguration,
   		but its absence will not trigger autoconfiguration.
+		If specified and DHCP is used, the user provided hostname will
+		be carried in the DHCP request to hopefully update DNS record.
 
   		Default: Client IP address is used in ASCII notation.