docs: move explanations into short module headers

Non-obvious context sits in a brief header at the top of a module, everything else uses single-line comments. niri binds use spawn-sh strings, so neon only overrides the command.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

modules/nixos/appimage.nix

@@ -1,17 +1,10 @@
+# Running an AppImage directly fails on NixOS: its runtime dlopens
+# libfuse.so.2, which nix-ld doesn't cover. appimage-run unpacks the image
+# into an FHS sandbox instead. Used for the BitBox02 wallet from bitbox.swiss.
+
 {
-  # AppImages ship a squashfs payload that the bundled runtime mounts through
-  # libfuse.so.2 at startup. nix-ld resolves the ELF interpreter but not that
-  # dlopen, so running one directly dies with "dlopen(): error loading
-  # libfuse.so.2". appimage-run sidesteps the mount entirely: it unpacks the
-  # image and runs the payload inside an FHS sandbox, which the bundled Qt and
-  # electron style binaries need anyway.
-  #
-  # Used for the BitBox02 wallet AppImage, downloaded fresh from
-  # https://bitbox.swiss/download/ rather than installed from nixpkgs.
   programs.appimage = {
     enable = true;
-    # register AppImages with binfmt_misc so ./Something.AppImage runs as-is,
-    # no appimage-run prefix to remember.
-    binfmt = true;
+    binfmt = true; # ./Something.AppImage runs without the appimage-run prefix
   };
 }