Update docs for aws_security_group and aws_security_group_rule

Author: mattburgess

website/docs/r/security_group.html.markdown

@@ -236,6 +236,8 @@ The following arguments are required:
 
 The following arguments are optional:
 
+~> **Note** Although `cidr_blocks`, `ipv6_cidr_blocks`, `prefix_list_ids`, and `security_groups` are all marked as optional, you _must_ provide one of them in order to configure the source of the traffic.
+
 * `cidr_blocks` - (Optional) List of CIDR blocks.
 * `description` - (Optional) Description of this ingress rule.
 * `ipv6_cidr_blocks` - (Optional) List of IPv6 CIDR blocks.
@@ -254,6 +256,8 @@ The following arguments are required:
 
 The following arguments are optional:
 
+~> **Note** Although `cidr_blocks`, `ipv6_cidr_blocks`, `prefix_list_ids`, and `security_groups` are all marked as optional, you _must_ provide one of them in order to configure the destination of the traffic.
+
 * `cidr_blocks` - (Optional) List of CIDR blocks.
 * `description` - (Optional) Description of this egress rule.
 * `ipv6_cidr_blocks` - (Optional) List of IPv6 CIDR blocks.

website/docs/r/security_group_rule.html.markdown

@@ -99,6 +99,8 @@ or `egress` (outbound).
 
 The following arguments are optional:
 
+~> **Note** Although `cidr_blocks`, `ipv6_cidr_blocks`, `prefix_list_ids`, and `source_security_group_id` are all marked as optional, you _must_ provide one of them in order to configure the source of the traffic.
+
 * `cidr_blocks` - (Optional) List of CIDR blocks. Cannot be specified with `source_security_group_id` or `self`.
 * `description` - (Optional) Description of the rule.
 * `ipv6_cidr_blocks` - (Optional) List of IPv6 CIDR blocks. Cannot be specified with `source_security_group_id` or `self`.