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.
The Terraform configuration syntax defines what arrays are.
Use the word array consistently throughout the documentation
instead of list.
The corresponding JSON datatype is called array as well, and
since the Terraform configuration syntax is interoperable with
JSON it makes sense to use the term array to describe them.
