Updating header comments with expected behavior

gtests/net/packetdrill/tests/bsd/tcp/listen/listen-incoming-syn-rst.pkt

-// Test behavior when a listener gets an incoming packet that has
-// the SYN and RST bits set.
+// Test behavior when a listener gets an incoming packet from new
+// sender on a non-established session that has the SYN and RST bits set.
+// The listener should send a RESET.
+// Checking if we can establish a connection with the client
+// after a connection reset.
 
 0.000 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
 0.000 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0