We talked about this in the latest Documentation Workgroup meeting and Joe and I also talked about this separately.
For visibility I thought that I'd repost the same information / brief summary of that conversation here.
I personally don't know—and if I ever knew I have since forgotten—if there was ever an intention to support documentation of snippets that doesn't also contain documentation about some library or executable that the snippets are about.
That said, I'm not sure if that documentation setup would be likely and it if would be particularly useful to developers. If the snippets are about a library or executable, why wouldn't they be part of the documentation for that library or executable?
When Joe and talked about this, Joe brought up The Swift Programming Language (TSPL) as a unique example where this could be relevant because it's not about a library. However, we agreed that in this case snippets could easily be supporter by adding a "main" symbol graph file without any symbols and using that module page as the root of the documentation.
As far as I know and can tell, it seems very sensible to formalize that that a snippet symbol graph is interpreted as an extension onto another symbol graph.