Clarify authorization prompt can show after starting extraction

[zorgiepoo]

Misc Checklist

• [x] My change requires a documentation update on Sparkle's website repository
• [ ] My change requires changes to generate_appcast, generate_keys, or sign_update

Testing

I tested and verified my change by using one or multiple of these methods:

• [ ] Sparkle Test App
• [ ] Unit Tests
• [ ] My own app
• [ ] Other (please specify)

NA

macOS version tested: [place version here]