Clarify syntax for RDS maintenance window param

Closes #2069

website/source/docs/providers/aws/r/db_instance.html.markdown

@@ -57,6 +57,8 @@ The following arguments are supported:
 * `iops` - (Optional) The amount of provisioned IOPS. Setting this implies a
     storage_type of "io1".
 * `maintenance_window` - (Optional) The window to perform maintenance in.
+  Syntax: "ddd:hh24:mi-ddd:hh24:mi". Eg: "Mon:00:00-Mon:03:00".
+  See [RDS Maintenance Window docs](http://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/AdjustingTheMaintenanceWindow.html) for more.
 * `multi_az` - (Optional) Specifies if the RDS instance is multi-AZ
 * `port` - (Optional) The port on which the DB accepts connections.
 * `publicly_accessible` - (Optional) Bool to control if instance is publicly accessible.