doc: mention pm is not enforced when using fd

PR-URL: #53125
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Tierney Cyren <hello@bnb.im>
Reviewed-By: Marco Ippolito <marcoippolito54@gmail.com>
Reviewed-By: Ulises Gascón <ulisesgascongonzalez@gmail.com>

doc/api/permissions.md

@@ -580,6 +580,8 @@ There are constraints you need to know before using this system:
   not subject to the rules of the Permission Model.
 * OpenSSL engines cannot be requested at runtime when the Permission
   Model is enabled, affecting the built-in crypto, https, and tls modules.
+* Using existing file descriptors via the `node:fs` module bypasses the
+  Permission Model.
 
 #### Limitations and Known Issues