docs(resource/role_mapping): add link to reference of available role IDs (#919)

When creating `mongodbatlas_federated_settings_org_role_mapping`
resources, one might wonder what are the available identifiers for the
`roles` field. This adds a link to their reference.

Author: Horgix

website/docs/r/federated_settings_org_role_mapping.html.markdown

@@ -43,6 +43,8 @@ resource "mongodbatlas_federated_settings_org_role_mapping" "org_group_role_mapp
 * `role_assignments` - Atlas roles and the unique identifiers of the groups and organizations associated with each role.
 * `group_id` - Unique identifier of the project to which you want the role mapping to apply.
 * `roles` - Specifies the Roles that are attached to the Role Mapping.
+  Available role IDs can be found on [the User Roles
+  Reference](https://www.mongodb.com/docs/atlas/reference/user-roles/).
 
 ## Attributes Reference
 
@@ -57,4 +59,4 @@ FederatedSettingsOrgRoleMapping can be imported using federation_settings_id-org
 $ terraform import mongodbatlas_federated_settings_org_role_mapping.org_group_role_mapping_import 6287a663c7f7f7f71c441c6c-627a96837f7f7f7e306f14-628ae97f7f7468ea3727
 ```
 
-For more information see: [MongoDB Atlas API Reference.](https://www.mongodb.com/docs/atlas/reference/api/federation-configuration/)
+For more information see: [MongoDB Atlas API Reference.](https://www.mongodb.com/docs/atlas/reference/api/federation-configuration/)