add document for TCP OFO, PAWS and skip ACK counters

add document and examples for below counters:
TcpExtTCPOFOQueue
TcpExtTCPOFODrop
TcpExtTCPOFOMerge
TcpExtPAWSActive
TcpExtPAWSEstab
TcpExtTCPACKSkippedSynRecv
TcpExtTCPACKSkippedPAWS
TcpExtTCPACKSkippedSeq
TcpExtTCPACKSkippedFinWait2
TcpExtTCPACKSkippedTimeWait
TcpExtTCPACKSkippedChallenge

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