adding exception list for notification_group_fields in monitor resour…

…ce docs

website/docs/r/monitor.html.markdown

@@ -348,7 +348,7 @@ The following arguments are supported:
 - `group_notifications` - (Optional) Whether or not to group notifications for individual items that meet the trigger condition. Defaults to true.
 - `playbook` - (Optional - Beta) Notes such as links and instruction to help you resolve alerts triggered by this monitor. {{Markdown}} supported. It will be enabled only if available for your organization. Please contact your Sumo Logic account team to learn more.
 - `alert_name` - (Optional) The display name when creating alerts. Monitor name will be used if `alert_name` is not provided. All template variables can be used in `alert_name` except `{{AlertName}}`, `{{AlertResponseURL}}`, `{{ResultsJson}}`, and `{{Playbook}}`.
-- `notification_group_fields` - (Optional) The set of fields to be used to group alerts and notifications for a monitor. The value of this field will be considered only when 'groupNotifications' is true.
+- `notification_group_fields` - (Optional) The set of fields to be used to group alerts and notifications for a monitor. The value of this field will be considered only when 'groupNotifications' is true. The fields with very high cardinality such as `_raw`, `_messagetime`, `_receipttime`, and `_messageid` are not allowed for Alert Grouping.
 - `obj_permission` - (Optional) `obj_permission` construct represents a Permission Statement associated with this Monitor. A set of `obj_permission` constructs can be specified under a Monitor. An `obj_permission` construct can be used to control permissions Explicitly associated with a Monitor. But, it cannot be used to control permissions Inherited from a Parent / Ancestor. Default FGP would be still set to the Monitor upon creation (e.g. the creating user would have full permission), even if no `obj_permission` construct is specified at a Monitor and the FGP feature is enabled at the account.
     - `subject_type` - (Required) Valid values:
         - `role`