fixup(cloudwatch_log_group) documentation (#12193)

retention_in_days actually has fixed values, it's not an arbitrary integer.

see https://docs.aws.amazon.com/AmazonCloudWatchLogs/latest/APIReference/API_PutRetentionPolicy.html#API_PutRetentionPolicy_RequestSyntax

website/docs/r/cloudwatch_log_group.html.markdown

@@ -30,7 +30,7 @@ The following arguments are supported:
 * `name` - (Optional, Forces new resource) The name of the log group. If omitted, Terraform will assign a random, unique name.
 * `name_prefix` - (Optional, Forces new resource) Creates a unique name beginning with the specified prefix. Conflicts with `name`.
 * `retention_in_days` - (Optional) Specifies the number of days
-  you want to retain log events in the specified log group.
+  you want to retain log events in the specified log group.  Possible values are: 1, 3, 5, 7, 14, 30, 60, 90, 120, 150, 180, 365, 400, 545, 731, 1827, and 3653. 
 * `kms_key_id` - (Optional) The ARN of the KMS Key to use when encrypting log data. Please note, after the AWS KMS CMK is disassociated from the log group,
 AWS CloudWatch Logs stops encrypting newly ingested data for the log group. All previously ingested data remains encrypted, and AWS CloudWatch Logs requires
 permissions for the CMK whenever the encrypted data is requested.