[docs] Advise contributors to check for truncated PR titles

[asb]

GitHub will use the first line of the commit as the title for a single-commit PR, but truncates it at 72 characters https://github.com/orgs/community/discussions/12450. This truncation makes the PRs less readable if not manually undone, and even worse, the truncated form may survive through to commit if "Squash and rebase" is used in the GitHub web UI. From preparing LLVM Weekly, I've seen this a number of times and it really does make it more annoying to flick through commits.

I'm not sure if this is the best place for the guidance, or whether you get the same behaviour when creating a PR with gh, but I'm quite keen we give a warning of some sort about this behaviour.

[joker-eph]

What about having our bots check for this pattern and advise the user to fix it?

This is also a problem of reviewers not flagging it by the way.

[asb]

This is also a problem of reviewers not flagging it by the way.

That's a fair point - I think it's made a bit more difficult by the fact the relationship between the commit message and the PR is less clear than before. e.g. someone who lands their commit by updating the branch then merging it from the git CLI would never see this issue, even if they left GitHub's UI to mangle the commit title for their PR.

[tstellar]

What about having our bots check for this pattern and advise the user to fix it?

I don't think this would be hard to implement, but I'm fine with the documentation change until someone implements this.

[preames]

LGTM

I agree this is a definite anti-feature in github, and warning about it for now is a good step.