add documents for snmp counters

Add explaination of below counters:
TcpExtTCPRcvCoalesce
TcpExtTCPAutoCorking
TcpExtTCPOrigDataSent
TCPSynRetrans
TCPFastOpenActiveFail
TcpExtListenOverflows
TcpExtListenDrops
TcpExtTCPHystartTrainDetect
TcpExtTCPHystartTrainCwnd
TcpExtTCPHystartDelayDetect
TcpExtTCPHystartDelayCwnd

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