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

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

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

PR-URL: https://github.com/nodejs/node/pull/34611


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