Update doc for postgresql_grant_role (#151)

Author: cyrilgdn

website/docs/r/postgresql_grant_role.html.markdown

@@ -14,8 +14,6 @@ When using ``postgresql_grant_role`` resource it is likely because the PostgreSQ
 
 ~> **Note:** This resource needs PostgreSQL version 9 or above.
 
-~> **Note:** `postgresql_grant_role` **cannot** be used in conjunction with `postgresql_role` or they will fight over what your role grants should be.
-
 ## Usage
 
 ```hcl
@@ -26,6 +24,24 @@ resource "postgresql_grant_role" "grant_root" {
 }
 ```
 
+~> **Note:** If you use `postgresql_grant_role` for a role that you also manage with a `postgresql_role` resource, you need to ignore the changes of the `roles` attribute in the `postgresql_role` resource or they will fight over what your role grants should be. e.g.:
+```hcl
+resource "postgresql_role" "bob" {
+  role = "bob"
+
+  lifecycle {
+    ignore_changes = [
+      roles,
+    ]
+  }
+}
+
+resource "postgresql_grant_role" "bob_admin" {
+  role       = "bob"
+  grant_role = "admin"
+}
+```
+
 ## Argument Reference
 
 * `role` - (Required) The name of the role that is granted a new membership.