docs: improve bounds_to_vertices and vertices_to_bounds docstrings

[SAY-5]

Addresses the docstring numpydoc warnings in #322 for the two public helpers functions: single-line summary (SS06), a description for the return value of vertices_to_bounds (RT03), and a See Also section plus runnable Examples (SA01, EX01) for both functions. The issue notes it "would be good to clean up these warnings".

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.07%. Comparing base (a9cebee) to head (4f0c556).
⚠️ Report is 125 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #642      +/-   ##
==========================================
+ Coverage   85.78%   87.07%   +1.29%     
==========================================
  Files          13       16       +3     
  Lines        2364     3320     +956     
  Branches      183      309     +126     
==========================================
+ Hits         2028     2891     +863     
- Misses        303      381      +78     
- Partials       33       48      +15     

Flag	Coverage Δ
mypy	46.23% <ø> (+7.69%)	⬆️
unittests	92.57% <ø> (-1.42%)	⬇️

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