mergecap: improve the example.

Explain, in detail, exactly what it's trying to do and, for each of the
three commands in the example, what each step does, as well as
explaining what the calculation using the end time of one capture and
start time of another capture is doing.

(Where did this example come from?  What is the real-world goal of this
exercise?  And why is it an example in which all the fancy stuff is done
in commands *other* than mergecap?)

doc/mergecap.pod

@@ -150,17 +150,34 @@ This setting is mandatory.
 
 =head1 EXAMPLES
 
-To merge two capture files together, 100 seconds apart use:
+To merge two capture files together into a third capture file, in which
+the last packet of one file arrives 100 seconds before the first packet
+of another file, use the following sequence of commands.
+
+First, use:
 
     capinfos -aeS a.pcap b.pcap
 
-(Let's suppose a.pcap starts at 1009932757 and b.pcap ends
-at 873660281. 1009932757 - 873660281 - 100 = 136272376
-seconds.)
+to determine the start and end times of the two capture files, as
+seconds since January 1, 1970, 00:00:00 UTC.
+
+If a.pcap starts at 1009932757 and b.pcap ends at 873660281, then the
+time adjustment to b.pcap that would make it end 100 seconds before
+a.pcap begins would be 1009932757 - 873660281 - 100 = 136272376 seconds.
+
+Thus, the next step would be to use:
 
     editcap -t 136272376 b.pcap b-shifted.pcap
+
+to generate a version of b.pcap with its time stamps shifted 136272376
+ahead.
+
+Then the final step would be to use :
+
     mergecap -w compare.pcap a.pcap b-shifted.pcap
 
+to merge a.pcap and the shifted b.pcap into compare.pcap.
+
 =head1 SEE ALSO
 
 pcap(3), wireshark(1), tshark(1), dumpcap(1), editcap(1), text2pcap(1),