docs: clarify Firestore auth context event fields

[SSDWGG]

Summary

• clarify that Firestore *WithAuthContext triggers expose event.authType and event.authId
• add a short JSDoc description for FirestoreAuthEvent
• keep the change documentation-only; no runtime behavior changes

Fixes #1706

Testing

• Not run; documentation-only change.

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[gemini-code-assist]

Code Review

This pull request updates the JSDoc documentation in src/v2/providers/firestore.ts to clarify the usage of the authentication context (event.authType and event.authId) in Firestore event triggers and the FirestoreAuthEvent interface. There are no review comments, so I have no feedback to provide.