code: public static <T> Mono<T> fromCompletionStage(CompletionStage<? extends T> completionStage) {
  		return onAssembly(new MonoCompletionStage<>(completionStage));
  	}
docs: Create a {@link Mono}, producing its value using the provided {@link CompletionStage}.
  
  <p>
  <img class="marble" src="doc-files/marbles/fromFuture.svg" alt="">
  <p>
  Note that the completion stage is not cancelled when that Mono is cancelled, but
  that behavior can be obtained by using {@link #doFinally(Consumer)} that checks
  for a {@link SignalType#CANCEL} and calls eg.
  {@link CompletionStage#toCompletableFuture() .toCompletableFuture().cancel(false)}.
  
  @param completionStage {@link CompletionStage} that will produce a value (or a null to
  complete immediately)
  @param <T> type of the expected value
  @return A {@link Mono}.