Document `auth_string` output parameter for google_redis_instance (#1… #2422

Job	Run time
terraform-provider-google / generate-repository	3m 27s
docs-examples / generate-repository	48s
terraform-provider-google-beta / generate-repository	3m 19s
terraform-google-conversion / generate-repository	1m 7s
terraform-google-conversion / compile-and-unit-tests	8m 12s
terraform-provider-google-beta / unit-test-terraform-provider-google-beta	9m 15s
terraform-provider-google / unit-test-terraform-provider-google	5m 50s
	31m 58s

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