doc: update fs.watch() availability for IBM i

Add a line that IBM i does not support `fs.watch()`.

Fixes: #34607
Refs: https://nodejs.org/docs/latest/api/fs.html#fs_availability

PR-URL: #34611
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/fs.md

@@ -3996,6 +3996,7 @@ to be notified of filesystem changes.
 * 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.
+* On IBM i systems, this feature is not supported.
 
 If the underlying functionality is not available for some reason, then
 `fs.watch()` will not be able to function and may thrown an exception.