docs: add panic conditions documentation for SetMode and Redirect.Render

[xingzihai]

This PR adds documentation for panic conditions in the following functions:

1. SetMode (mode.go): Documents that it panics if the value is not a valid gin mode (debug, release, or test).

2. Redirect.Render (render/redirect.go): Documents that it panics if the status code is not a valid redirect status code (300-308, except 201).

This is a continuation of the panic conditions documentation effort started in the previous PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.43%. Comparing base (3dc1cd6) to head (0fd0eab).
⚠️ Report is 275 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4608      +/-   ##
==========================================
- 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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