docs(engine): note why TryFromJs omits f32

[AayushMainali-Github]

Summary

• Document that TryFromJs intentionally omits f32 because JS numbers are f64 and the f32 cast is lossy
• Point users at f64 + explicit as f32, #[boa(from_js_with = "...")] for derives, or JsValue::to_f32 for JS-style coercion
• Follow-up to the discussion in feat(engine): support f32 TryFromJs #5444 / TryFromJs is not implemented for f32 #5360

Test plan

• Confirm the new docs render correctly via cargo doc -p boa_engine --no-deps
• Skim the wording against the rationale in feat(engine): support f32 TryFromJs #5444

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	53,125	53,125	0
Passed	51,073	51,073	0
Ignored	1,482	1,482	0
Failed	570	570	0
Panics	0	0	0
Conformance	96.14%	96.14%	0.00%

Tested main commit: 4fc75c6ae9d85f2b8065c6716f88e9b35318438c
Tested PR commit: 61821e3e8c3dd292558e4c7c82032faf0067fe0a
Compare commits: 4fc75c6...61821e3

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 62.88%. Comparing base (6ddc2b4) to head (61821e3).
⚠️ Report is 1012 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5455       +/-   ##
===========================================
+ Coverage   47.24%   62.88%   +15.63%     
===========================================
  Files         476      530       +54     
  Lines       46892    59102    +12210     
===========================================
+ Hits        22154    37165    +15011     
+ Misses      24738    21937     -2801     

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