document limitation of instance id / network interface id

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

@@ -27,6 +27,11 @@ The following arguments are supported:
 * `instance` - (Optional) EC2 instance ID.
 * `network_interface` - (Optional) Network interface ID to associate with.
 
+~> **NOTE:** You can specify either the `instance` ID or the `network_interface` ID, 
+but not both. Including both will **not** return an error from the AWS API, but will
+have undefined behavior. See the relevant [AssociateAddress API Call][1] for
+more information.
+
 ## Attributes Reference
 
 The following attributes are exported:
@@ -36,3 +41,5 @@ The following attributes are exported:
 * `instance` - Contains the ID of the attached instance.
 * `network_interface` - Contains the ID of the attached network interface.
 
+
+[1]: http://docs.aws.amazon.com/fr_fr/AWSEC2/latest/APIReference/API_AssociateAddress.html