As a follow-up to #10666, this is a request to promote the following no-document-title message from WARNING to INFO:

[WARNING] This document format requires a nonempty <title> element.
  Defaulting to 'some_document.md' as the title.
  To specify a title, use 'title' in metadata or --metadata title="...".

The current behavior is a common-sense fallback behavior. Currently, the only way to suppress this message is to use --quiet, which then also suppresses other potentially more concerning warning messages.