provider/aws: doc EBS block device mix-match caveats

refs hashicorp#5706

Author: phinze

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

@@ -109,6 +109,8 @@ Each `ebs_block_device` supports the following:
 
 Modifying any `ebs_block_device` currently requires resource replacement.
 
+~> **NOTE on EBS block devices:** If you use `ebs_block_device` on an `aws_instance`, Terraform will assume management over the full set of non-root EBS block devices for the instance, and treats additional block devices as drift. For this reason, `ebs_block_device` cannot be mixed with external `aws_ebs_volume` + `aws_ebs_volume_attachment` resources for a given instance.
+
 Each `ephemeral_block_device` supports the following:
 
 * `device_name` - The name of the block device to mount on the instance.

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

@@ -11,6 +11,8 @@ description: |-
 Provides an AWS EBS Volume Attachment as a top level resource, to attach and
 detach volumes from AWS Instances.
 
+~> **NOTE on EBS block devices:** If you use `ebs_block_device` on an `aws_instance`, Terraform will assume management over the full set of non-root EBS block devices for the instance, and treats additional block devices as drift. For this reason, `ebs_block_device` cannot be mixed with external `aws_ebs_volume` + `aws_ebs_volume_attachment` resources for a given instance.
+
 ## Example Usage
 
 ```