draft-ietf-opsawg-pcap.md: Add some information on byte order

[fxlb]

As in the pcap-savefile libpcap manual page.

[fxlb]

It's already been said differently. closing. "All fields in the File Header and in the headers of Packet Records will always be written according to the characteristics (little endian / big endian) of the machine that is writing the file."