book: make it clear that auth channel is not needed for signing

[conradoplg]

Closes #820

[github-actions]

Visit the preview URL for this PR (updated for commit a4ec6b2):

https://frost-book--pr953-docs-auth-channel-x99vrd79.web.app

_{(expires Mon, 05 Jan 2026 13:26:03 GMT)}

_{🔥 via Firebase Hosting GitHub Action 🌎}

_{Sign: b7f43333fbf8fa5527ab3f0dd7bd10bb1fa7401c}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.63%. Comparing base (1882937) to head (a4ec6b2).
⚠️ Report is 13 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #953      +/-   ##
==========================================
+ Coverage   77.46%   77.63%   +0.16%     
==========================================
  Files          41       41              
  Lines        4300     4332      +32     
==========================================
+ Hits         3331     3363      +32     
  Misses        969      969              

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[natalieesk]

Just a typo fix

[mergify]

Merge Queue Status

✅ The pull request has been merged at a4ec6b2

This pull request spent 4 minutes 6 seconds in the queue, including 32 seconds running CI.
The checks were run on draft #977.

Required conditions to merge

• #approved-reviews-by >= 1 [🛡 GitHub branch protection]
  • book: make it clear that auth channel is not needed for signing #953
• #changes-requested-reviews-by = 0 [🛡 GitHub branch protection]
  • book: make it clear that auth channel is not needed for signing #953
• #review-threads-unresolved = 0 [🛡 GitHub branch protection]
  • book: make it clear that auth channel is not needed for signing #953
• base=main