docs: clarify comment spacing after the final import - #2717

Closes #2009.

Clarify that lines_after_imports can add a blank line before a comment following the final import even when ensure_newline_before_comments is false. Add a verified example and a cross-link to lines_after_imports. Explain that isort’s Black profile does not run Black, and that running Black separately adds the blank line in this example.

Update both the options documentation and its generator. Sorting behavior is unchanged.

Validation

On Windows 11 with Python 3.13.5:

• Verified the isort and Black example outputs.
• Checked that the updated documentation section matches its generator.
• Built Sphinx HTML with warnings treated as errors, with external font and intersphinx fetching disabled. Checked rendered examples and the cross-link.
• Ran the repository cleanup and test scripts: 668 passed, 5 skipped.
• Ruff lint and format checks passed.
• git diff --check passed.

The standard documentation build encountered network/SSL errors fetching external fonts. The done.sh wrapper could not invoke nested Bash on this Windows setup, so its clean.sh and test.sh components were run explicitly through Git Bash. Unrelated pre-existing differences between the generated options page and its generator were left unchanged.

Codex assisted with the change and ran the checks reported above.