doc: clarify fs.link and fs.linkSync arguments

Clarifies documentation by replacing the argument names `srcpath`
and `dstpath` with more descriptive `existingPath` and `newPath`,
reflecting how POSIX describes `link()`.

PR-URL: https://github.com/nodejs/node/pull/9145


Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>