Handle rejection not found in docs

[msolano00]

The provided example is missing the user-not-found handler:

passport.use(new CustomStrategy(
  function(req, done) {
    User.findOne({
      username: req.body.username
    }, function (err, user) {
      done(err, user);
    });
  }
));

I understand that this is already stated in other strategies but it would be great to have something like:

passport.use(new CustomStrategy(
  function(req, done) {
    User.findOne({
      username: req.body.username
    }, function (err, user) {
      if (typeof user !== 'undefined' && user) done(err, user);
      else done(err, false);    // <- This
    });
  }
));

[mbell8903]

Go ahead and add a PR for the docs update!!