docs(tvix/nix-compat): mention Directory::close on Node::directory

Change-Id: I19e6a3b63eb1b1609b96544b2529d71234bcb13b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9613
Reviewed-by: flokli <flokli@flokli.de>
Autosubmit: edef <edef@edef.eu>
Tested-by: BuildkiteCI

tvix/nix-compat/src/nar/writer/mod.rs

@@ -129,6 +129,9 @@ impl<'a, 'w> Node<'a, 'w> {
 
     /// Make this node a directory, the content of which is set using the
     /// resulting [`Directory`] value.
+    ///
+    /// It is the caller's responsibility to invoke [`Directory::close`],
+    /// or invalid archives will be produced silently.
     pub fn directory(mut self) -> io::Result<Directory<'a, 'w>> {
         self.write(&wire::TOK_DIR)?;
         Ok(Directory::new(self))