Add update-browserslist-db mention on --update-db call and deprecatio…

…n hint. (#791)

* add --update-db key to the usage hint

The --update-db key is missing in the usage hint printed to the terminal when CLI is called with the --help key.

* add --update-db depracation hint

And rollback the --update-db print in the help hint.

cli.js

@@ -38,6 +38,8 @@ if (isArg('--help') || isArg('-h')) {
 } else if (isArg('--version') || isArg('-v')) {
   process.stdout.write('browserslist ' + pkg.version + '\n')
 } else if (isArg('--update-db')) {
+  process.stdout.write('The --update-db command is deprecated. Please use npx update-browserslist-db@latest instead.\n')
+  process.stdout.write('Browserslist DB update will still be made.\n')
   /* c8 ignore next 3 */
   updateDb(function (str) {
     process.stdout.write(str)