man: Add a section to pcap_loop.3pcap. [skip ci]

As discussed on tcpdump-workers, move a paragraph to a new BACKWARD
COMPATIBILITY section and tell a specific libpcap version, also make
the formatting consistent.

Author: infrastation

pcap_loop.3pcap

@@ -17,7 +17,7 @@
 .\" WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF
 .\" MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
 .\"
-.TH PCAP_LOOP 3PCAP "25 July 2018"
+.TH PCAP_LOOP 3PCAP "22 August 2020"
 .SH NAME
 pcap_loop, pcap_dispatch \- process packets from a live capture or savefile
 .SH SYNOPSIS
@@ -92,21 +92,6 @@ will return 0, even when not in non-blocking mode, as there are no
 packets to process.  Applications should be prepared for this to happen,
 but must not rely on it happening.
 .PP
-.ft B
-(In older versions of libpcap, the behavior when
-\fIcnt\fP
-was
-.B 0
-was undefined; different platforms and devices behaved
-differently, so code that must work with older versions of libpcap
-should use
-.BR \-1 ,
-not
-.BR 0 ,
-as the value of
-\fIcnt\fP.)
-.ft R
-.PP
 .I callback
 specifies a
 .I pcap_handler
@@ -220,5 +205,18 @@ or
 may be called with
 .I p
 as an argument to fetch or display the error text.
+.SH BACKWARD COMPATIBILITY
+.PP
+In libpcap versions before 1.5.0, the behavior when
+.I cnt
+was
+.B 0
+was undefined; different platforms and devices behaved differently,
+so code that must work with these versions of libpcap should use
+.BR \-1 ,
+not
+.BR 0 ,
+as the value of
+.IR cnt .
 .SH SEE ALSO
 .BR pcap (3PCAP)