doc: add heading for permission model limitations

These limitations are not specific to the file system, so they should
not be listed in the "File System Permissions" section.

PR-URL: https://github.com/nodejs/node/pull/47989
Reviewed-By: Rafael Gonzaga <rafael.nunu@hotmail.com>
Reviewed-By: Mestery <mestery@protonmail.com>
Reviewed-By: Deokjin Kim <deokjin81.kim@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

doc/api/permissions.md

@@ -539,6 +539,8 @@ Wildcards are supported too:
 * `--allow-fs-read=/home/test*` will allow read access to everything
   that matches the wildcard. e.g: `/home/test/file1` or `/home/test2`
 
+#### Limitations and known issues
+
 There are constraints you need to know before using this system:
 
 * Native modules are restricted by default when using the Permission Model.