providers/aws: Clarifies db_security_group usage.
providers/aws: Clarifies db_security_group usage.

db_security_group is only intended to be used in EC2-Classic Platform.
For DB instances in a VPC, we associate VPC security groups instead,
when declaring the db_instance resource.

terraform-providers/terraform-provider-aws,Ninir/terraform-provider-aws,terraform-providers/terraform-provider-aws,ashinohara/terraform-provider-aws,ashinohara/terraform-provider-aws,Ninir/terraform-provider-aws,terraform-providers/terraform-provider-aws,Ninir/terraform-provider-aws,ClementCunin/terraform-provider-aws,kjmkznr/terraform-provider-aws,kjmkznr/terraform-provider-aws,ClementCunin/terraform-provider-aws,Ninir/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,terraform-providers/terraform-provider-aws,ClementCunin/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,ClementCunin/terraform-provider-aws,nbaztec/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,nbaztec/terraform-provider-aws,ashinohara/terraform-provider-aws,nbaztec/terraform-provider-aws,ashinohara/terraform-provider-aws,nbaztec/terraform-provider-aws,kjmkznr/terraform-provider-aws,kjmkznr/terraform-provider-aws