IGMP: Document igmp_link_local_mcast_reports

Document the addition of a new sysctl variable which controls the
generation of IGMP reports for link local multicast groups in the
224.0.0.X range.

IGMP reports for local multicast groups can now be optionally
inhibited by setting the value to zero e.g.:
echo 0 > /proc/sys/net/ipv4/igmp_link_local_mcast_reports

To retain backwards compatibility the previous behaviour is retained
by default on system boot or reverted by setting the value back to
non-zero.

Signed-off-by: Philip Downey <pdowney@brocade.com>
Signed-off-by: David S. Miller <davem@davemloft.net>