Add notification messages to hccap2john/hccapx2john about pcap being prefered

[magnumripper]

See #4243. We can use hccap/hccapx files, but normally I'd recommend using wpapcap2john if you do have a pcap at all. That, or (possibly even better) something from hcxtools which we could also mention (Zerobeat's hcxdumptool is the absolute best sniffer for WPA as well).