Improve -Q documentation

The `-Q, --squiet=SECS` option does not replace the summary
statistics of `-q, --quiets`, but adds interval summary
statistics every SECS seconds to the output.

doc/fping.pod

@@ -178,7 +178,8 @@ show ICMP error messages.
 
 =item B<-Q>, B<--squiet>=I<SECS>
 
-Like B<-q>, but show summary results every n seconds.
+Like B<-q>, but additionally show interval summary results every I<SECS>
+seconds.
 
 =item B<-r>, B<--retry>=I<N>
 

src/fping.c

@@ -2939,7 +2939,7 @@ void usage(int is_error)
     fprintf(out, "   -N, --netdata      output compatible for netdata (-l -Q are required)\n");
     fprintf(out, "   -o, --outage       show the accumulated outage time (lost packets * packet interval)\n");
     fprintf(out, "   -q, --quiet        quiet (don't show per-target/per-ping results)\n");
-    fprintf(out, "   -Q, --squiet=SECS  same as -q, but show summary every n seconds\n");
+    fprintf(out, "   -Q, --squiet=SECS  same as -q, but add interval summary every SECS seconds\n");
     fprintf(out, "   -s, --stats        print final stats\n");
     fprintf(out, "   -u, --unreach      show targets that are unreachable\n");
     fprintf(out, "   -v, --version      show version\n");