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]: https://github.com/signalapp/Signal-Desktop/blob/d3d2b0ec529ffd5ad74deb17831005a1acacc949/Gruntfile.js#L222
[1]: https://github.com/electron/asar