The detail that really pets me backwards is the whole "if you're here, you already know what this is for and how to use it" vibe. It's not just Tarsnap; many aspects of the OSS world uphold this most unfortunate tradition. I believe with all of my heart that it is unnecessarily hostile and antisocial.
Many of the founders and teams I've mentored over the years have experienced me spending a large amount of time and effort explaining the importance of clearly stating why a thing exists and why you should use it; how it works and how it will not create additional unknown risks; what tools it is intended to replace... as the barest civil minimum.
This is just my opinion, but it's a hill I'd die on if necessary.