[docs] Add More Info When Windows Code-Signing is Done After Tauri Build

[elibroftw]

I use SSL.Com esigner github action. One gotcha is that the sig file generated by Tauri is unintentionally invalidated by code signing the generated MSIs. To make matters worse, the updater plugin doesn't log or inform me by default that the sig file was invalid and is the reason update is stuck at 100% downloaded.

Proposals:

1. Update code signing docs on how to sign after Tauri build action and still have functioning auto-updating
2. Update updater plugin (or docs) to log that the sig verficiation failed. Even throwing downloadAndInstall would be great.