doc: copyedit fs.watch() information

Removed "Only FSEvents supports this type of file watching so it is
unlikely any additional platforms will be added soon."

Per @saghul, "FSEvents" refers to the OSX API, but since we added
Windows support it may not be  unlikely that we add more.

PR-URL: https://github.com/nodejs/node/pull/3097


Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Saúl Ibarra Corretgé <saghul@gmail.com>