ipconfig: Document setting of NIS domain name

ic_do_bootp_ext() is responsible for parsing the "ip=" and "nfsaddrs="
kernel parameters. If a "." character is found in parameter 4 (the
client's hostname), everything before the first "." is used as the
hostname, and everything after it is used as the NIS domain name (but
not necessarily the DNS domain name).

Document this behaviour in Documentation/filesystems/nfs/nfsroot.txt,
as it is not made explicit.
Signed-off-by: NChris Novakovic <chris@chrisn.me.uk>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/filesystems/nfs/nfsroot.txt

@@ -123,10 +123,13 @@ ip=<client-ip>:<server-ip>:<gw-ip>:<netmask>:<hostname>:<device>:<autoconf>:
 
 		Default:  Determined using autoconfiguration.
 
-  <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.
+  <hostname>	Name of the client. If a '.' character is present, anything
+		before the first '.' is used as the client's hostname, and anything
+		after it is used as its NIS domain name. May be supplied by
+		autoconfiguration, but its absence will not trigger autoconfiguration.
+		If specified and DHCP is used, the user-provided hostname (and NIS
+		domain name, if present) will be carried in the DHCP request; this
+		may cause a DNS record to be created or updated for the client.
 
   		Default: Client IP address is used in ASCII notation.