docs: Add document to constant AuthProxyUserKey and BasicAuthForProxy.

[EndlessParadox1]

Just a little fix.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.18%. Comparing base (3dc1cd6) to head (dad3a1d).
Report is 22 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3887      +/-   ##
==========================================
- Coverage   99.21%   99.18%   -0.03%     
==========================================
  Files          42       43       +1     
  Lines        3182     2701     -481     
==========================================
- Hits         3157     2679     -478     
+ Misses         17       12       -5     
- Partials        8       10       +2     

Flag	Coverage Δ
	?
-race	∅ <ø> (?)
-tags "sonic avx"	99.17% <ø> (?)
-tags go_json	99.17% <ø> (?)
-tags nomsgpack	99.17% <ø> (?)
go-1.18	99.11% <ø> (-0.01%)	⬇️
go-1.19	99.18% <ø> (-0.03%)	⬇️
go-1.20	99.18% <ø> (-0.03%)	⬇️
go-1.21	99.18% <ø> (-0.03%)	⬇️
go-1.22	99.18% <ø> (?)
macos-latest	99.18% <ø> (-0.03%)	⬇️
ubuntu-latest	99.18% <ø> (-0.03%)	⬇️

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