Improve durable consumer unsubscribe comment (#1100)

Signed-off-by: Sylvain Rabot <sylvain@abstraction.fr>

js.go

@@ -67,9 +67,11 @@ type JetStream interface {
 	// gone.
 	// * If Durable() option is specified, the library will attempt to lookup a JetStream
 	// consumer with this name, and if found, will bind to it and not attempt to
-	// delete it. However, if not found, the library will send a request to create
-	// such durable JetStream consumer. The library will delete the JetStream consumer
-	// after an Unsubscribe() or Drain().
+	// delete it. However, if not found, the library will send a request to
+	// create such durable JetStream consumer. Note that the library will delete
+	// the JetStream consumer after an Unsubscribe() or Drain() only if it
+	// created the durable consumer while subscribing. If the durable consumer
+	// already existed prior to subscribing it won't be deleted.
 	// * If Bind() option is provided, the library will attempt to lookup the
 	// consumer with the given name, and if successful, bind to it. If the lookup fails,
 	// then the Subscribe() call will return an error.