docs: clarify how partial clones are handled

[rolandjitsu]

What kind of change does this PR introduce?

• fix
• feat
• refactor
• perf
• docs
• test
• build / ci
• chore

Summary

Clarify how we handle partial clones.

Tests

• Added / updated tests (unit, plus integration where it fits)
• Not relevant, because: it's just a comment update.

Checklist

• CI is green locally: cargo fmt --all --check, cargo clippy --all-targets --all-features --locked -- -D warnings, cargo test --all-features
• Commits follow Conventional Commits; AI-assisted commits carry an Assisted-by: trailer (see CONTRIBUTING.md / AGENTS.md)
• Preserves the read-only, pull-only invariant (no push or proactive replication to upstream)
• Docs / README updated if behavior or flags changed

Breaking change?

N/A.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.69%. Comparing base (7a99407) to head (fe16b4c).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #38   +/-   ##
=======================================
  Coverage   96.69%   96.69%           
=======================================
  Files           6        6           
  Lines        1846     1846           
=======================================
  Hits         1785     1785           
  Misses         61       61           

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