doc: improve scaleway_instance_security_group_rules doc (#513)

Author: jerome-quere

website/docs/r/instance_security_group_rules.html.markdown

@@ -9,6 +9,10 @@ description: |-
 
 Creates and manages Scaleway Compute Instance security group rules. For more information, see [the documentation](https://developers.scaleway.com/en/products/instance/api/#security-groups-8d7f89).
 
+This resource can be used to externalize rules from a `scaleway_instance_security_group` to solve circular dependency problems. When using this resource do not forget to set `external_rules = true` on the security group.
+
+~> **Warning:** In order to guaranty rules order in a given security group only one scaleway_instance_security_group_rules is allowed per security group.
+
 ## Examples
 
 ### Basic