docs: Add x-ref for ordering messages

docs: Clarify subscription expiration policy

PiperOrigin-RevId: 511270484

Source-Link: googleapis/googleapis@4dea519

Source-Link: googleapis/googleapis-gen@8dbb4db
Copy-Tag: eyJwIjoiYXBpcy9Hb29nbGUuQ2xvdWQuUHViU3ViLlYxLy5Pd2xCb3QueWFtbCIsImgiOiI4ZGJiNGRiYWRlNWEzYTkwY2MzODM5ZGQyNTk3NmE4YzBhNjc1Mjg2In0=

apis/Google.Cloud.PubSub.V1/Google.Cloud.PubSub.V1/Pubsub.g.cs

@@ -1472,6 +1472,8 @@ public sealed partial class PubsubMessage : pb::IMessage<PubsubMessage>
     /// delivered to subscribers in the order in which they are received by the
     /// Pub/Sub system. All `PubsubMessage`s published in a given `PublishRequest`
     /// must specify the same `ordering_key` value.
+    /// For more information, see [ordering
+    /// messages](https://cloud.google.com/pubsub/docs/ordering).
     /// </summary>
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
     [global::System.CodeDom.Compiler.GeneratedCode("protoc", null)]
@@ -4883,7 +4885,8 @@ public sealed partial class Subscription : pb::IMessage<Subscription>
     /// successfully consuming messages from the subscription or is issuing
     /// operations on the subscription. If `expiration_policy` is not set, a
     /// *default policy* with `ttl` of 31 days will be used. The minimum allowed
-    /// value for `expiration_policy.ttl` is 1 day.
+    /// value for `expiration_policy.ttl` is 1 day. If `expiration_policy` is set,
+    /// but `expiration_policy.ttl` is not set, the subscription never expires.
     /// </summary>
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
     [global::System.CodeDom.Compiler.GeneratedCode("protoc", null)]