📚 DOCS: Document renderer constructor differences

[user01010111]

Document that MarkdownIt passes itself to renderer_cls, unlike the argument-free JavaScript renderer constructor. Clarify the custom constructor contract and that RendererHTML accepts but ignores an optional parser argument.

Closes #127.

Validation: 981 tests passed; all pre-commit checks passed; strict docs build passed on Python 3.10 with Sphinx 8.1.3. Sphinx 9 fails in the unchanged API-doc configuration on both upstream and this branch.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.85%. Comparing base (bff75ed) to head (935bc0f).
⚠️ Report is 9 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #421   +/-   ##
=======================================
  Coverage   95.85%   95.85%           
=======================================
  Files          64       64           
  Lines        3619     3619           
=======================================
  Hits         3469     3469           
  Misses        150      150           

Flag	Coverage Δ
pytests	95.85% <ø> (ø)

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[chrisjsewell]

thanks