docs: document use_component_rect caveat, simplify api

[ccbrown]

What It Does

Does a little bit of follow-up polish for use_component_rect.

Related Issues

#145

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.06%. Comparing base (9dd54e7) to head (fbcbf69).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #174      +/-   ##
==========================================
- Coverage   89.15%   89.06%   -0.09%     
==========================================
  Files          34       34              
  Lines        5827     5826       -1     
  Branches     5827     5826       -1     
==========================================
- Hits         5195     5189       -6     
- Misses        520      526       +6     
+ Partials      112      111       -1     

Files with missing lines	Coverage Δ
packages/iocraft/src/hooks/use_component_rect.rs	97.56% <100.00%> (+2.32%)	⬆️

... and 1 file with indirect coverage changes

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