docs: Clarify aws_instance.public_ip w/ EIP behavior

Closes #1425

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

@@ -137,7 +137,7 @@ The following attributes are exported:
 * `key_name` - The key name of the instance
 * `public_dns` - The public DNS name assigned to the instance. For EC2-VPC, this 
  is only available if you've enabled DNS hostnames for your VPC
-* `public_ip` - The public IP address assigned to the instance, if applicable.
+* `public_ip` - The public IP address assigned to the instance, if applicable. **NOTE**: If you are using an [`aws_eip`](/docs/providers/aws/r/eip.html) with your instance, you should refer to the EIP's address directly and not use `public_ip`, as this field will change after the EIP is attached.
 * `private_dns` - The private DNS name assigned to the instance. Can only be 
  used inside the Amazon EC2, and only available if you've enabled DNS hostnames 
  for your VPC