fix: preserve paragraphs in recommended spelling descriptions

[skwh54]

Fixes #398.

Summary

Preserve spaces and paragraph boundaries when rendering multiline recommended_spelling descriptions as Verso content.

Consecutive non-empty source lines are joined with spaces, while blank lines produce separate paragraphs. Existing behavior for absent and single-line descriptions remains unchanged.

Root cause

The additional information was split into lines and combined using String.join, which inserts no separator. This concatenated adjacent words and discarded paragraph boundaries.

Lean's string representation preserves line breaks and blank lines when generating Markdown. This change mirrors the resulting soft-wrap and paragraph structure using Verso blocks.

Verification

Rendered the reproduction from the linked issue using both the baseline and patched versions of doc-gen4. The Lean input was identical in both builds.

Before

<p>First paragraph spansmultiple source lines.Second paragraph.</p>

After

<p>First paragraph spans multiple source lines.</p>
<p>Second paragraph.</p>

Both documentation builds completed successfully with Lean v4.33.0-rc1, and the generated pages were served locally over HTTP and verified in Chromium.