specify the duration format for the Pub/Sub message retention duratio…

…n (#9400) (#6599)

When passing a string that doesn't end with an `s`, get the error message

> Invalid value at 'topic.message_retention_duration' (type.googleapis.com/google.protobuf.Duration), Field 'messageRetentionDuration', Illegal duration format; duration must end with 's'

Made the wording similar to the [KMS key rotation period](https://registry.terraform.io/providers/hashicorp/google/latest/docs/resources/kms_crypto_key#rotation_period) description.
[upstream:80ee4d4b7d0529189700b26aabb604e07e19198d]

Signed-off-by: Modular Magician <[email protected]>

.changelog/9400.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+pubsub: clarified the expected format for the `message_retention_duration`
+```

google-beta/services/pubsub/resource_pubsub_topic.go

@@ -90,7 +90,8 @@ the last messageRetentionDuration are always available to subscribers.
 For instance, it allows any attached subscription to seek to a timestamp
 that is up to messageRetentionDuration in the past. If this field is not
 set, message retention is controlled by settings on individual subscriptions.
-Cannot be more than 31 days or less than 10 minutes.`,
+The rotation period has the format of a decimal number, followed by the
+letter 's' (seconds). Cannot be more than 31 days or less than 10 minutes.`,
 			},
 			"message_storage_policy": {
 				Type:     schema.TypeList,

website/docs/r/pubsub_topic.html.markdown

@@ -157,7 +157,8 @@ The following arguments are supported:
   For instance, it allows any attached subscription to seek to a timestamp
   that is up to messageRetentionDuration in the past. If this field is not
   set, message retention is controlled by settings on individual subscriptions.
-  Cannot be more than 31 days or less than 10 minutes.
+  The rotation period has the format of a decimal number, followed by the
+  letter `s` (seconds). Cannot be more than 31 days or less than 10 minutes.
 
 * `project` - (Optional) The ID of the project in which the resource belongs.
     If it is not provided, the provider project is used.