ipvs: Document sysctl pmtu_disc

Document sysctl pmtu_disc based on commit 3654e61137db ("ipvs: add
pmtu_disc option to disable IP DF for TUN packets").

Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Simon Horman <horms@verge.net.au>
1 file changed