Added docs about m3.medium to tests

builtin/providers/aws/resource_aws_instance_test.go

@@ -473,6 +473,10 @@ const testAccInstanceConfigBlockDevices = `
 resource "aws_instance" "foo" {
 	# us-west-2
 	ami = "ami-55a7ea65"
+
+	# In order to attach an encrypted volume to an instance you need to have an
+	# m3.medium or larger. See "Supported Instance Types" in:
+	# http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/EBSEncryption.html
 	instance_type = "m3.medium"
 
 	root_block_device {