[chore] document how set flag works, and current limitations

[bogdandrutu]

Updates #6294

[bogdandrutu]

/cc @codeboten @dmitryax look at the limitations, let me know if you are ok with them, and also think if we have more.

[codecov]

Codecov Report

Base: 92.06% // Head: 92.06% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (29a0b69) compared to base (ee02bc7).
Patch coverage: 100.00% of modified lines in pull request are covered.

❗ Current head 29a0b69 differs from pull request most recent head 057ebc5. Consider uploading reports for the commit 057ebc5 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6321      +/-   ##
==========================================
- Coverage   92.06%   92.06%   -0.01%     
==========================================
  Files         219      219              
  Lines       13242    13241       -1     
==========================================
- Hits        12191    12190       -1     
  Misses        828      828              
  Partials      223      223              

Impacted Files	Coverage Δ
service/flags.go	100.00% <100.00%> (ø)

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