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

[Tom Stellard via llvm-commits]

tstellar wrote:

> 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.


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