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

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.

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