Plugins packaged with the app is one of the gsoc ideas and basically could solve all the prompts to user about backups. So, no warning or reminder would be needed ("best code is no code").
Regardless, to summarize the point you've raised:
Things unique to Joplin are best to disclose to new users
It is a broader, long running topic. The forum itself has been serving a place for that kind of information. Thanks to team, we're already getting nice, professional guides and how-to's on the most pressing issues (like Restoring Deleted Notes).
Easing the access to docs (and forum indirectly) is also a one of the gsoc ideas. So maybe the work of linking to them in the app via "learn more" or strategic on-hover-info-bubbles could be a test of the final doc structure.