Merge pull request hashicorp#4139 from hashicorp/aws-sg-doc-update

providers/aws: Update Security Group docs

Author: catsby

website/source/docs/providers/aws/r/security_group.html.markdown

@@ -66,7 +66,8 @@ resource "aws_security_group" "allow_all" {
 
 The following arguments are supported:
 
-* `name` - (Required) The name of the security group
+* `name` - (Optional) The name of the security group. If omitted, Terraform will
+assign a random, unique name
 * `description` - (Optional) The security group description. Defaults to "Managed by Terraform". Cannot be "".
 * `ingress` - (Optional) Can be specified multiple times for each
    ingress rule. Each ingress block supports fields documented below.