defaultIndex setting description fix - its really a data view! (#147730)

## Summary

The defaultIndex setting title and description were poor. I've updated
them to correctly reference data views and be more specific about where
they're used.

Closes #143609

(cherry picked from commit b697704)

src/plugins/data/server/ui_settings.ts

@@ -164,12 +164,12 @@ export function getUiSettings(): Record<string, UiSettingsParams<unknown>> {
     },
     defaultIndex: {
       name: i18n.translate('data.advancedSettings.defaultIndexTitle', {
-        defaultMessage: 'Default index',
+        defaultMessage: 'Default data view',
       }),
       value: null,
       type: 'string',
       description: i18n.translate('data.advancedSettings.defaultIndexText', {
-        defaultMessage: 'The index to access if no index is set',
+        defaultMessage: 'Used by discover and visualizations when a data view is not set.',
       }),
       schema: schema.nullable(schema.string()),
     },