(Accessibility) Page size doesnt read instructions

[aphelionz]

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

[elasticmachine]

Pinging @elastic/es-ui

[PhilippBaranovskiy]

Tested with NVDA, it works as expected.

It doesn't work with ChromeVox Chrome extension since the last one doesn't support correctly aria-labelled and aria-describedby attributes.
The Google developers says that ChromeVox Classic doesn't support aria-describedby, but ChromeVox Next does.

Unfortunately, Chrome extenstion is backported ChromeVox Classic version:

while ChromeVox Next is just a operation system program that comes installed on Chromebooks.
Therefore, I suggest that we shouldn't take ChromeVox results into account as well as switching from ChromeVox extenstion to NVDA unless we could use a Chromebook for that purpose.

cc @bhavyarm