doc: warn about relying on fs gc close behavior

Warn about using the bahvior where file handles are closed automatically
when the file is closed.

PR-URL: #27972
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

doc/api/fs.md

@@ -3733,9 +3733,14 @@ added: v10.0.0
 
 A `FileHandle` object is a wrapper for a numeric file descriptor.
 Instances of `FileHandle` are distinct from numeric file descriptors
-in that, if the `FileHandle` is not explicitly closed using the
-`filehandle.close()` method, they will automatically close the file descriptor
+in that they provide an object oriented API for working with files.
+
+If a `FileHandle` is not closed using the
+`filehandle.close()` method, it might automatically close the file descriptor
 and will emit a process warning, thereby helping to prevent memory leaks.
+Please do not rely on this behavior in your code because it is unreliable and
+your file may not be closed. Instead, always explicitly close `FileHandle`s.
+Node.js may change this behavior in the future.
 
 Instances of the `FileHandle` object are created internally by the
 `fsPromises.open()` method.