doc: path.format provide more examples

This change was to add upon the algorithm description of path.format
by adding examples for unix systems that clarified behavior in
various scenarios.

PR-URL: https://github.com/nodejs/node/pull/5838


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Roman Klauke <romaaan.git@gmail.com>