signal-desktop: Document the build "expiration"

This adds a comment why it's important to always backport any updates as
suggested in #68232. I've als included a few more details to document
the behaviour.

Sources:
- Default build lifetime [0]
- asar - Electron Archive [1]

[0]: d3d2b0ec52/Gruntfile.js (L222)
[1]: https://github.com/electron/asar

pkgs/applications/networking/instant-messengers/signal-desktop/default.nix

@@ -58,7 +58,13 @@ let
 
 in stdenv.mkDerivation rec {
   pname = "signal-desktop";
-  version = "1.27.2";
+  version = "1.27.2"; # Please backport all updates to the stable channel.
+  # All releases have a limited lifetime and "expire" 90 days after the release.
+  # When releases "expire" the application becomes unusable until an update is
+  # applied. The expiration date for the current release can be extracted with:
+  # $ grep -a "^{\"buildExpiration" "${signal-desktop}/libexec/resources/app.asar"
+  # (Alternatively we could try to patch the asar archive, but that requires a
+  # few additional steps and might not be the best idea.)
 
   src = fetchurl {
     url = "https://updates.signal.org/desktop/apt/pool/main/s/signal-desktop/signal-desktop_${version}_amd64.deb";