[stacked] Document stacked generated diff layout

[barrettruth]

Part of #358.

Goal

Document stacked generated diff layout accurately and update any dual-rail-specific help text.

Scope

• Update :Gdiff and :Greview signatures to include stacked.
• Explain that stacked is a generated single-column diffs:// layout with one context-aware line-number rail.
• Document exact line-number semantics:
  • - lines display old-side line numbers;
  • + lines display new-side line numbers;
  • context lines display new-side line numbers;
  • headers/metadata do not display file line numbers.
• Include a small ASCII example that makes repeated numbers around replacements unsurprising.
• Update layout parameter descriptions.
• Update highlight group wording where current text is dual-rail-specific.

Non-goals

• Do not document a public config option unless one is intentionally added in another issue.
• Do not describe Pierre-specific web internals.

Acceptance criteria

• Vimdoc explains ++layout=stacked for both :Gdiff and :Greview.
• The docs make clear that stacked reuses generated unified diff text and differs by rail style.
• Highlight group descriptions remain accurate for both dual and single rails.

Verification

• Run vimdoc formatting/check commands if available in the repo workflow.
• Run the relevant docs/lint target from the project workflow.