doc: clarify eventType in fs.watch

'rename' is confusing, and it's not clear what "they" refers to.

Fixes: https://github.com/nodejs/node/issues/9082
PR-URL: https://github.com/nodejs/node/pull/9318


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>