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

[Mehdi Amini via llvm-commits]

joker-eph wrote:

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.

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