Add more "-v" detail to tcpdump(1) for GH #380. [skip ci]

Capture what Guy has figured out so far.

tcpdump.1.in

@@ -20,7 +20,7 @@
 .\" WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF
 .\" MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
 .\"
-.TH TCPDUMP 1  "29 Aug 2020"
+.TH TCPDUMP 1  "15 September 2020"
 .SH NAME
 tcpdump \- dump traffic on a network
 .SH SYNOPSIS
@@ -810,7 +810,10 @@ When writing to a file with the
 .B \-w
 option and at the same time not reading from a file with the
 .B \-r
-option, report, once per second, the number of packets captured.
+option, report to stderr, once per second, the number of packets captured. In
+Solaris, FreeBSD and possibly other operating systems this periodic update
+currently can cause loss of captured packets on their way from the kernel to
+tcpdump.
 .TP
 .B \-vv
 Even more verbose output.