docs: show the Depends on form the conventions check accepts

CONTRIBUTING.md told authors to write `Depends on #N` in the
description, but the pr-template rule in tools/check_conventions.py
only accepts the checklist line `Depends on: #N` or `Depends on: none`
from the pull request template. A pull request that followed the
document failed the Conventions job with "fill Depends on with #N or
none". The sentence now shows the checklist form.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01V9UU71SP1UKGR51Sowbgmq

CONTRIBUTING.md

@@ -53,7 +53,7 @@ The [pull request template](.github/pull_request_template.md) is the checklist f
 - Open pull requests per author are limited by how many of theirs are merged into `main`: 1 with none merged, 20 with at least 1, no limit from 20. A pull request opened or reopened over the limit is closed automatically; reopen it once another one is merged or closed.
 - Before starting, check that no open pull request already implements the same functions.
 - Keep the branch up to date with `main` and resolve conflicts yourself; rebasing and force-pushing is fine.
-- If a pull request needs another one first, say so in the description (`Depends on #N`).
+- If a pull request needs another one first, say so on the checklist line (`Depends on: #N`; `none` otherwise).
 - Run the tests before opening the pull request and describe what was tested (OS, title or homebrew).
 - Investigation notes, reports, screenshots and logs go in the pull request, not in the repository. Images for documentation go in the [gist](https://gist.github.com/boykopovar/0e53f2e1426f29ecd41e3b51540b8a90) comments.
 - Say whether the change was written with AI assistance. The author of the pull request is responsible for every line of it.