Document and better enforce main thread only requirement for using Sparkle methods.

In general, Sparkle usage should be constrained to main thread only.

Continuation of #2746.

Misc Checklist

• 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)

Code review, tested test app. No errors should be logged in normal cases.

macOS version tested: 15.5 (24F74)