Add comments regarding tab-index=-1

src/components/views/messages/DateSeparator.js

@@ -57,6 +57,7 @@ export default class DateSeparator extends React.Component {
 
     render() {
         // ARIA treats <hr/>s as separators, here we abuse them slightly so manually treat this entire thing as one
+        // tab-index=-1 to allow it to be focusable but do not add tab stop for it, primarily for screen readers
         return <h2 className="mx_DateSeparator" role="separator" tabIndex={-1}>
             <hr role="none" />
             <div>{ this.getLabel() }</div>