add quick info about data sources to upgrade guide (#2510)
add quick info about data sources to upgrade guide (#2510)

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/cc @danawillow
terraform-providers/terraform-provider-google,terraform-providers/terraform-provider-google,terraform-providers/terraform-provider-google,ubschmidt2/terraform-provider-google,hashicorp/terraform-provider-google,hashicorp/terraform-provider-google,terraform-providers/terraform-provider-google,terraform-providers/terraform-provider-google,ubschmidt2/terraform-provider-google,hashicorp/terraform-provider-google,ubschmidt2/terraform-provider-google,ubschmidt2/terraform-provider-google,hashicorp/terraform-provider-google