docs: comments on update_rewards assertions

[noa-starkware]

Note

Clarifies comments in update_rewards about staker existence/activation window and non-zero balance timing.

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This change is 

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.74%. Comparing base (53bea2b) to head (92fdb6d).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #96   +/-   ##
=======================================
  Coverage   95.74%   95.74%           
=======================================
  Files          42       42           
  Lines       10041    10041           
=======================================
  Hits         9614     9614           
  Misses        427      427           

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[NirLevi-starkware]

@NirLevi-starkware reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @noa-starkware)