net: add doc for ip_early_demux sysctl

commit 6648bd7e (ipv4: Add sysctl knob to control
early socket demux) introduced such sysctl, but forgot to add
doc into Documentation/networking/ip-sysctl.txt. This patch adds it.

Basically I grab the doc from the description of commit 41063e9d
(ipv4: Early TCP socket demux.) and the above commit.

Cc: Eric Dumazet <edumazet@google.com>
Cc: Alexander Duyck <alexander.h.duyck@intel.com>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Cong Wang <amwang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>