Document --update-interval capture option better in help messages.

Wireshark supports it as well, so add it to ui/commandline.c.

Also, note that the number specified is in units of milliseoncds.

dumpcap.c

@@ -500,7 +500,7 @@ print_usage(FILE *output)
     fprintf(output, "  -L, --list-data-link-types\n");
     fprintf(output, "                           print list of link-layer types of iface and exit\n");
     fprintf(output, "  --list-time-stamp-types  print list of timestamp types for iface and exit\n");
-    fprintf(output, "  --update-interval        interval between updates with new packets (def: %dms)\n", DEFAULT_UPDATE_INTERVAL);
+    fprintf(output, "  --update-interval        interval between updates with new packets, in milliseconds (def: %dms)\n", DEFAULT_UPDATE_INTERVAL);
     fprintf(output, "  -d                       print generated BPF code for capture filter\n");
     fprintf(output, "  -k <freq>,[<type>],[<center_freq1>],[<center_freq2>]\n");
     fprintf(output, "                           set channel on wifi interface\n");

tshark.c

@@ -456,7 +456,7 @@ print_usage(FILE *output)
     fprintf(output, "  -L, --list-data-link-types\n");
     fprintf(output, "                           print list of link-layer types of iface and exit\n");
     fprintf(output, "  --list-time-stamp-types  print list of timestamp types for iface and exit\n");
-    fprintf(output, "  --update-interval        interval between updates with new packets (def: %dms)\n", DEFAULT_UPDATE_INTERVAL);
+    fprintf(output, "  --update-interval        interval between updates with new packets, in milliseconds (def: %dms)\n", DEFAULT_UPDATE_INTERVAL);
     fprintf(output, "\n");
     fprintf(output, "Capture stop conditions:\n");
     fprintf(output, "  -c <packet count>        stop after n packets (def: infinite)\n");

ui/commandline.c

@@ -99,6 +99,7 @@ commandline_print_usage(bool for_help_option) {
     fprintf(output, "  -L, --list-data-link-types\n");
     fprintf(output, "                           print list of link-layer types of iface and exit\n");
     fprintf(output, "  --list-time-stamp-types  print list of timestamp types for iface and exit\n");
+    fprintf(output, "  --update-interval        interval between updates with new packets, in milliseconds (def: %dms)\n", DEFAULT_UPDATE_INTERVAL);
     fprintf(output, "\n");
     fprintf(output, "Capture display:\n");
     fprintf(output, "  -k                       start capturing immediately (def: do nothing)\n");