docs: add two limits of scaled headings, and the width docs README.ja.md missed

[delphinus]

Docs only.

Known limitations of scaled headings

Two entries, in README.md, README.ja.md and both help files:

• A heading that a float covers stays plain until the float closes. This is the behaviour fix(text-size): leave a heading plain while a float covers it #75 introduced to fix Scaled headings are drawn over floating windows that cover them #72.
• Fractionally scaled headings assume the font fits the cell grid (Uneven spacing and clipped glyphs in fractionally scaled headings #65). Runs are sized in whole cells from the characters' cell widths, while Kitty lays the text out with the font's own advances, so a CJK fallback narrower than two cells shows gaps at run boundaries and Latin glyphs can be clipped at the end of a run. With SF Mono Square, whose CJK glyphs are exactly two cells, the reproduction from Uneven spacing and clipped glyphs in fractionally scaled headings #65 shows neither; :MdRender textsize off is the way out for fonts that do.

README.ja.md

#73 documented g:md_render_max_width and width=N in README.md but not in README.ja.md. It gets the same ### 描画幅 section, the note under the command table and the updated completion sentence.

Checks

• The new help lines, rendered with tree-sitter on: within 78 columns, and the vimdoc parser reads every code span in them (no backtick right after 、).
• ai-words diff.

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