docs: document panic conditions in Handle, StaticFS, and Bind

[Folium1]

Pull Request Checklist

Please ensure your pull request meets the following requirements:

• Open your pull request against the master branch.
• All tests pass in available continuous integration systems (e.g., GitHub Actions).
• Tests are added or modified as needed to cover code changes.
• If the pull request introduces a new feature, the feature is documented in the docs/doc.md.

Thank you for contributing!

What this PR does

Adds panic documentation to three exported functions whose doc comments
don't mention that they can panic on invalid input:

• RouterGroup.Handle — panics on invalid HTTP method
• RouterGroup.StaticFS — panics when relativePath contains : or *
• Bind — panics when val is a pointer

Callers relying on godoc have no way to discover these crash conditions
without reading the source.

Changes

Documentation only — no behavior or logic changes.

• routergroup.go: added // It panics if ... to Handle and StaticFS
• utils.go: added // It panics if ... to Bind

Why

Go convention (and the standard library itself) documents panic conditions
in doc comments. For example, regexp.MustCompile says
"It panics if the expression cannot be parsed." These three functions
follow the same pattern but lack the documentation.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.37%. Comparing base (3dc1cd6) to head (f83a7d1).
⚠️ Report is 275 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4595      +/-   ##
==========================================
- Coverage   99.21%   98.37%   -0.84%     
==========================================
  Files          42       48       +6     
  Lines        3182     3138      -44     
==========================================
- Hits         3157     3087      -70     
- Misses         17       42      +25     
- Partials        8        9       +1     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags sonic	98.36% <ø> (?)
-tags go_json	98.29% <ø> (?)
-tags nomsgpack	98.35% <ø> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.25	98.37% <ø> (?)
go-1.26	98.37% <ø> (?)
macos-latest	98.37% <ø> (-0.84%)	⬇️
ubuntu-latest	98.37% <ø> (-0.84%)	⬇️

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