[llvm] [docs] Advise contributors to check for truncated PR titles (PR #68021)

[Alex Bradbury via llvm-commits]

asb wrote:

> 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.

https://github.com/llvm/llvm-project/pull/68021