doc: clarify options for fs.watchFile()

The explanation for the `options` argument to `fs.watchFile()` had
missing punctuation. I took the opportunity to try to rewrite the
paragraph for greater clarity.

PR-URL: https://github.com/nodejs/node/pull/2425


Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Brendan Ashworth <brendan.ashworth@me.com>