Steps to reproduce (assumes ChromeVox or similar)

1. Navigate to Rollup Jobs UI
2. Click Create Rollup Job
3. Tab to the page size field

Actual Result

4. The label is read but not the instructions like the other fields

Expected Result

4. Screen reader should read "A larger page size will roll up data quicker, but requires more memory."

Meta Issue #26862

Kibana Version: 6.5

Relevant WCAG Criteria: 3.3.2 Labels or Instructions