docs: add panic conditions documentation for Handle, StaticFS, and Bind

[xingzihai]

Summary

This PR adds documentation for panic conditions in three exported functions:

• Handle: panics if httpMethod is not a valid HTTP method (uppercase ASCII letters only)
• StaticFS: panics if relativePath contains URL parameters (: or *)
• Bind: panics if val is a pointer

Motivation

Three exported public API functions contain panic() calls on invalid input, but none of the doc comments mention the panic conditions. Callers cannot discover the crash risk without reading the source. This is a documentation gap — the panics themselves are reasonable startup-time guards, but they should be documented.

Changes

Added panic condition documentation to each function's doc comment:

1. RouterGroup.Handle in routergroup.go
2. RouterGroup.StaticFS in routergroup.go
3. Bind in utils.go

Related Issue

Fixes #4594

Testing

This is a documentation-only change. No functional code changes were made.

• Documentation builds correctly
• No test changes needed (documentation only)

Checklist

• Documentation added/updated
• Commit message follows conventional commits
• PR title clearly describes the change

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.37%. Comparing base (3dc1cd6) to head (8f92e31).
⚠️ Report is 275 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4607      +/-   ##
==========================================
- 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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