doc: path functions ignore trailing slashes

Add notes about path.parse(), path.basename() and path.dirname()
ignoring trailing slashes.

PR-URL: https://github.com/nodejs/node/pull/12181
Fixes: https://github.com/nodejs/node/issues/6229


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>