docs(recovery): clarify RecoveryWithWriter only uses the first RecoveryFunc

[Gav1nnn]

Summary

Clarify the RecoveryWithWriter behavior when multiple RecoveryFunc values
are passed.

Gin currently only uses the first recovery handler from the variadic argument
list. This change documents that behavior and adds a test to prevent future
confusion.

Changes

• update the exported comment on RecoveryWithWriter
• add a test verifying only the first recovery handler is used

Fixes #4044

• gin/recovery.go:44
• gin/recovery_test.go:259

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.43%. Comparing base (3dc1cd6) to head (a6d98e9).
⚠️ Report is 275 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4610      +/-   ##
==========================================
- Coverage   99.21%   98.43%   -0.78%     
==========================================
  Files          42       48       +6     
  Lines        3182     3138      -44     
==========================================
- Hits         3157     3089      -68     
- Misses         17       40      +23     
- Partials        8        9       +1     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags sonic	98.42% <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	98.43% <ø> (?)
macos-latest	98.37% <ø> (-0.84%)	⬇️
ubuntu-latest	98.43% <ø> (-0.78%)	⬇️

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