net: add document for several snmp counters

add document for below counters:
TcpEstabResets
TcpAttemptFails
TcpOutRsts
TcpExtTCPSACKDiscard
TcpExtTCPDSACKIgnoredOld
TcpExtTCPDSACKIgnoredNoUndo
TcpExtTCPSackShifted
TcpExtTCPSackMerged
TcpExtTCPSackShiftFallback
TcpExtTCPWantZeroWindowAdv
TcpExtTCPToZeroWindowAdv
TcpExtTCPFromZeroWindowAdv
TcpExtDelayedACKs
TcpExtDelayedACKLocked
TcpExtDelayedACKLost
TcpExtTCPLossProbes
TcpExtTCPLossProbeRecovery

Signed-off-by: yupeng <yupeng0921@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>