chore: improve code comments clarity

[stringsbuilder]

improve code comments clarity

---

Note

Low Risk
Comment-only changes in order_builder and WebSocket client docs; no runtime behavior or API changes, so risk is minimal.

Overview
Improves documentation clarity by fixing typos/grammar in Rust doc comments.

Updates wording in to_fixed_u128 and several WebSocket subscription/unsubscribe method docs (e.g., “tick size changes”, “server unsubscribe request”) without changing any logic.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.54%. Comparing base (77264a4) to head (5abf64d).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #326   +/-   ##
=======================================
  Coverage   85.54%   85.54%           
=======================================
  Files          32       32           
  Lines        5167     5167           
=======================================
  Hits         4420     4420           
  Misses        747      747           

Flag	Coverage Δ
rust	85.54% <ø> (ø)

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