doc: immprove inode text in fs.md

Rewrite and edit somewhat confusing paragraph about inodes and AIX in
fs.md.

PR-URL: https://github.com/nodejs/node/pull/17519


Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>