Document some non-nullable Kotlin extensions can throw NoSuchElementException

Closes gh-31189

spring-messaging/src/main/kotlin/org/springframework/messaging/rsocket/RSocketRequesterExtensions.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2023 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -110,6 +110,7 @@ suspend fun RSocketRequester.RetrieveSpec.sendAndAwait() {
 /**
  * Coroutines variant of [RSocketRequester.RetrieveSpec.retrieveMono].
  *
+ * @throws NoSuchElementException if the underlying [Mono] does not emit any value
  * @author Sebastien Deleuze
  * @since 5.2
  */