[IPV4]: Updates to nfsroot documentation

The difference between ip=off and ip=::::::off has been a cause of much
confusion.  Document how each behaves, and do not contradict ourselves by
saying that "off" is the default when in fact "any" is the default and is
descibed as being so lower in the file.

Signed-off-by: Amos Waterland <apw@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>