pktgen: document all supported flags

The documentation misses a few of the supported flags. Fix this. Also
respect the dependency to CONFIG_XFRM for the IPSEC flag.

Cc: Fan Du <fan.du@windriver.com>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Mathias Krause <minipli@googlemail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>