Two things I don't like in your examples:
First, the default source changes depending on the target. In my opinion it would be more intuitive and simple if the default source is always HEAD. The documentation for --source in your link probably makes sense for a git developer, but not at all for a user. (Especially the part about its default value if absent).
Second, rename "--staged" to "--index".
So in summary, to update your examples, how about:
git restore --index file # reset the index from HEAD
git restore --worktree file # reset the worktree from HEAD
git restore --worktree --source=index file # reset the worktree from the index
git restore --index --worktree file # reset both the index and worktree from HEAD
git restore file # reset the worktree from HEAD