(Accessibility) Having to tab backward about "Create cron expression" is confusing

[aphelionz]

Steps to reproduce (assumes ChromeVox or similar)

1. Navigate to the Rollup Jobs UI
2. Click the Create Jobs Button
3. Tab to the create Cron Expression

Actual Result

4. User must tab backward to edit the field, with no indication that they must do so. Also, on a more minor note, it's unclear to a screen reader that "Learn more about cron expressions is a link."

Expected Result

4. Tab navigation of the Create Cron Expression elements should be apparent to the user. Perhaps auto-focus the field after the user clicks on Create Cron Expression?

Meta Issue #26862

Kibana Version: 6.5

Relevant WCAG Criteria: 3.2.2 On Input

[elasticmachine]

Pinging @elastic/es-ui