Give a more detailed explanation of the use of 0xa1b2c3d4 by rpcap. [skip ci]

0xa1b2c3d4 is used in the rpcap protocol - and, yes, I chose it because
it was already being used as a byte-order indication - but it doesn't
indicate any time between microsecond-time-resolution pcap format and
rpcap; any future changes to rpcap to support higher-resolution time
stamps, or pcapng-like capabilities, won't involve a change to the
byte-order indication value.

Author: guyharris

sf-pcap.c

@@ -71,7 +71,10 @@
 
 /*
  * Standard libpcap format.
- * rpcapd can too use this number in its network protocol to tell endianness.
+ *
+ * The same value is used in the rpcap protocol as an indication of
+ * the server byte order, to let the client know whether it needs to
+ * byte-swap some host-byte-order metadata.
  */
 #define TCPDUMP_MAGIC		0xa1b2c3d4