docs: document useHead behavior

[reslear]

Summary

• document the Ionic-specific useHead lifecycle behavior
• document support for calls outside component setup added in fix: support useHead outside of components #825
• replace the outdated useHead warning

Testing

• git diff --check
• pnpm docs:build (generation completes; existing unrelated minimark errors remain on two raw routes)

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 30.48%. Comparing base (6e52925) to head (1a00130).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #996   +/-   ##
=======================================
  Coverage   30.48%   30.48%           
=======================================
  Files          12       12           
  Lines         328      328           
  Branches       98       98           
=======================================
  Hits          100      100           
  Misses        228      228           

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