[BUG] - CronJob withSeconds documentation and implementation inconsistency

[rbroggi]

Describe the bug

The method CronJob documentation says that the 6th field is optional but it is actually mandatory (validation fails).

// CronJob defines a new job using the crontab syntax: `* * * * *`.
// An optional 6th field can be used at the beginning if withSeconds
// is set to true: `* * * * * *`.
// The timezone can be set on the Scheduler using WithLocation, or in the
// crontab in the form `TZ=America/Chicago * * * * *` or
// `CRON_TZ=America/Chicago * * * * *`
func CronJob(crontab string, withSeconds bool) JobDefinition {
	return cronJobDefinition{
		crontab:     crontab,
		withSeconds: withSeconds,
	}
}

The likely correction to this would be to change the following code block:

job.go:126

	if c.withSeconds {
		p := cron.NewParser(cron.Second | cron.Minute | cron.Hour | cron.Dom | cron.Month | cron.Dow | cron.Descriptor)
		cronSchedule, err = p.Parse(withLocation)
	} else {
		cronSchedule, err = cron.ParseStandard(withLocation)
	}

with:

	if c.withSeconds {
		p := cron.NewParser(cron.SecondOptional | cron.Minute | cron.Hour | cron.Dom | cron.Month | cron.Dow | cron.Descriptor)
		cronSchedule, err = p.Parse(withLocation)
	} else {
		cronSchedule, err = cron.ParseStandard(withLocation)
	}

To Reproduce

Steps to reproduce the behavior:

Attempt to create a CronJob with the following expression * * * * * using the withSeconds parameter true.

Version

v2.5.0

(latest)

Expected behavior

When invoking CronJob with withSeconds set to true, we should accept both seconds and non-seconds cron-expressions (as the documentation claims).

Additional context