doc: add note about fs.close undefined behavior

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
+though any other `fs` operation may lead to undefined behavior.
+
 ## fs.constants
 
 * {Object}