doc: add note about fs.close() about undefined behavior

Add notes to fs.close and fs.closeSync() about udnefined behavior.

PR-URL: #30966
Refs: #30864
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/fs.md

@@ -1533,6 +1533,9 @@ changes:
 Asynchronous close(2). No arguments other than a possible exception are given
 to the completion callback.
 
+Calling `fs.close()` on any file descriptor (`fd`) that is currently in use
+through any other `fs` operation may lead to undefined behavior.
+
 ## fs.closeSync(fd)
 <!-- YAML
 added: v0.1.21
@@ -1542,6 +1545,9 @@ added: v0.1.21
 
 Synchronous close(2). Returns `undefined`.
 
+Calling `fs.closeSync()` on any file descriptor (`fd`) that is currently in use
+through any other `fs` operation may lead to undefined behavior.
+
 ## fs.constants
 
 * {Object}