doc: mention pm is not enforced when using fd

Author: RafaelGSS

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.
+* The Permission Model is not upheld when utilizing existing file descriptors
+  via the `node:fs` module.
 
 #### Limitations and Known Issues