Update cockpit_source.md (#2900)

Author: jremy42

docs/resources/cockpit_source.md

@@ -36,7 +36,7 @@ This section lists the arguments that are supported:
 - `type` - (Required) The [type](https://www.scaleway.com/en/docs/observability/cockpit/concepts/#data-types) of data source. Possible values are: `metrics`, `logs`, or `traces`.
 - `region` - (Defaults to the region specified in the [provider configuration](../index.md#region)) The [region](../guides/regions_and_zones.md#regions) where the data source is located.
 - `project_id` - (Defaults to the Project ID specified in the [provider configuration](../index.md#project_id)) The ID of the Project the data source is associated with.
-- `retention_days` - (Optional, Defaults to 6) The number of days to retain data in the data source. Must be a value between 1 and 365. Changes to this field will force the creation of a new resource.
+- `retention_days` - (Required) The number of days to retain data in the data source. Must be a value between 1 and 365. For more details on retention policies, please refer to the [Scaleway Retention Documentation](https://www.scaleway.com/en/docs/cockpit/concepts/#retention). Note: Changes to this field will force the creation of a new resource.
 
 ## Attributes Reference