doc: add info on what's used for fswatch on AIX

Info is provided on for the other OS's. Add similar
level of info for AIX.

PR-URL: nodejs#6837
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>

doc/api/fs.md

@@ -1467,6 +1467,7 @@ to be notified of filesystem changes.
 * On OS X, this uses `kqueue` for files and 'FSEvents' for directories.
 * On SunOS systems (including Solaris and SmartOS), this uses `event ports`.
 * On Windows systems, this feature depends on `ReadDirectoryChangesW`.
+* On Aix systems, this feature depends on `AHAFS`, which must be enabled.
 
 If the underlying functionality is not available for some reason, then
 `fs.watch` will not be able to function. For example, watching files or