doc/build-helpers: fix appimage example where postExtract is used

doc/build-helpers/images/appimagetools.section.md

@@ -129,11 +129,16 @@ appimageTools.wrapType2 {
 The argument passed to `extract` can also contain a `postExtract` attribute, which allows you to execute additional commands after the files are extracted from the AppImage.
 `postExtract` must be a string with commands to run.
 
+:::{.warning}
+When specifying `postExtract`, you should use `appimageTools.wrapAppImage` instead of `appimageTools.wrapType2`.
+Otherwise `wrapType2` will extract the AppImage contents without respecting the `postExtract` instructions.
+:::
+
 :::{.example #ex-extracting-appimage-with-postextract}
 
 # Extracting an AppImage to install extra files, using `postExtract`
 
-This is a rewrite of [](#ex-extracting-appimage) to use `postExtract`.
+This is a rewrite of [](#ex-extracting-appimage) to use `postExtract` and `wrapAppImage`.
 
 ```nix
 { appimageTools, fetchurl }:
@@ -153,8 +158,10 @@ let
     '';
   };
 in
-appimageTools.wrapType2 {
-  inherit pname version src;
+appimageTools.wrapAppImage {
+  inherit pname version;
+
+  src = appimageContents;
 
   extraPkgs = pkgs: [ pkgs.at-spi2-core ];
 
@@ -164,6 +171,9 @@ appimageTools.wrapType2 {
     install -m 444 -D ${appimageContents}/usr/share/icons/hicolor/512x512/apps/irccloud.png \
       $out/share/icons/hicolor/512x512/apps/irccloud.png
   '';
+
+  # specify src archive for nix-update
+  passthru.src = src;
 }
 ```