In package comment, s/filename//
Everywhere: s/(path )?separator/slash/.

In Clean: /Dot-Dot right/ s/right/Right/.
In Dir: s/the all/all/
Avoid mention of "root directory":
    "If the path consists entirely of
    slashes, Dir returns a single slash. Otherwise the
    returned path does not end in a slash."

Add SplitList (like in path/filepath) for consistency.

Document which functions call Clean on their result (Join for one).

In Split: document that dir+file == path (there's a sentence in
path/filepath to lift).

Document ErrBadPattern.