docs: Document default values for the environment variables

[terrytangyuan]

Signed-off-by: terrytangyuan terrytangyuan@gmail.com

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[codecov]

Codecov Report

Merging #6087 (2612ae3) into master (eddd420) will decrease coverage by 0.06%.
The diff coverage is n/a.

❗ Current head 2612ae3 differs from pull request most recent head 0e02441. Consider uploading reports for the commit 0e02441 to get more accurate results

@@            Coverage Diff             @@
##           master    #6087      +/-   ##
==========================================
- Coverage   47.69%   47.63%   -0.07%     
==========================================
  Files         249      249              
  Lines       15737    15737              
==========================================
- Hits         7506     7496      -10     
- Misses       7295     7303       +8     
- Partials      936      938       +2     

Impacted Files	Coverage Δ
workflow/metrics/server.go	12.50% <0.00%> (-4.17%)	⬇️
cmd/argo/commands/get.go	56.45% <0.00%> (-1.94%)	⬇️
cmd/argoexec/commands/emissary.go	48.43% <0.00%> (-1.57%)	⬇️

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