Cron Preview screen needs context help that aids the user in writing cron expressions

Currently, the Cron expression in the Schedule section of Feed Summary provides no clue for what constitutes a valid expression. This is especially troublesome since there are multiple formats for cron including, but not limited to: Quartz Cron, Linux Crontab.

We should put an incon near the cron expression field that will popup a summary for help. e.g.

Cron Expression docs can be found at: http://www.quartz-scheduler.org/api/2.1.7/org/quartz/CronExpression.html

Perhaps a Summary of Options with a external link to the Java Docs for CronExpression would be sufficient. Here's a summary taken from java docs: