[DOCS] change wording on advanced settings UI on default index to default index pattern.

[TomonoriSoejima]

It took me a while to figure out what default index means until I landed on https://www.elastic.co/guide/en/kibana/7.17/index-patterns-api-default-set.html and understood that it is talking about index pattern.

Please consider to make this change to soften the confusion.

[preview] This functionality is in technical preview and may be changed or removed in a future release. Elastic will apply best effort to fix any issues, but features in technical preview are not subject to the support SLA of official GA features.Set a default index pattern ID. Kibana UI will use default index pattern unless user picks a different one. The API doesn’t validate if given index_pattern_id is a valid id.

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)

[elasticmachine]

Pinging @elastic/kibana-app-services (Team:AppServicesSv)

[afharo]

Apologies for the noise... it's actually owned by AppServices:

kibana/src/plugins/data/server/ui_settings.ts

Lines 167 to 177 in 5e57ffc

	defaultIndex: {
	name: i18n.translate('data.advancedSettings.defaultIndexTitle', {
	defaultMessage: 'Default index',
	}),
	value: null,
	type: 'string',
	description: i18n.translate('data.advancedSettings.defaultIndexText', {
	defaultMessage: 'The index to access if no index is set',
	}),
	schema: schema.nullable(schema.string()),
	},

[elasticmachine]

Pinging @elastic/kibana-data-discovery (Team:DataDiscovery)