Minor documentation hiccup

[rickb777]

In path.go, there is a blank line before func (a Afero) Walk(root string, walkFn filepath.WalkFunc) error so the documentation above isn't shown in godoc.

(v1.4.1)

[rickb777]

Also the second Walk function could do with something, e.g.

// Walk walks the file tree rooted at root - see details above.