Steps to reproduce (assumes ChromeVox or similar)

1. Open https://elastic.github.io/eui/#/layout/accordion in a new tab.
2. Tab through to the "Accordion content can dynamically change height" section and focus the "Increase Height" button
3. Press Enter

Actual Result

"Increase Height" is read repeatedly with no context as to what it is increasing, and to what

Expected Result

"Increase height to 3 items" => "Increase height to 4 items" etc, etc.

Category: #688: Elastic UI Buttons Accessibility
Relevant WCAG Criteria: [3.3.2 Input Assistance: Labels or Instructions(https://www.w3.org/WAI/WCAG20/quickref/#qr-minimize-error-cues)