Help snippets for special /command completions

[rolandwalker]

Description

Motivation: basic help is visible just from typing slash.

Unfortunately the help snippet is not as clearly visible when the user has

wider_completion_menu = True

in ~/.myclirc.

Example

Checklist

• I added this contribution to the changelog.md file.
• I added my name to the AUTHORS file (or it's already there).
• To lint and format the code, I ran

  uv run ruff check && uv run ruff format && uv run mypy --install-types .