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>

doc/api/fs.md

@@ -1701,8 +1701,12 @@ The listener callback gets two arguments `(eventType, filename)`.  `eventType` i
 `'rename'` or `'change'`, and `filename` is the name of the file which triggered
 the event.
 
-Please note the listener callback is attached to the `'change'` event
-fired by [`fs.FSWatcher`][], but they are not the same thing.
+Note that on most platforms, `'rename'` is emitted whenever a filename appears
+or disappears in the directory.
+
+Also note the listener callback is attached to the `'change'` event fired by
+[`fs.FSWatcher`][], but it is not the same thing as the `'change'` value of
+`eventType`.
 
 ### Caveats