[llvm] Clarify `lit`'s definition of failure and conditions when it exits with exit code 1 (PR #136190)

[via llvm-commits]

i-ky wrote:

Hi @jh7370!

> You might want to tweak the PR description given this revised version of the patch.

Sure, done.

> Otherwise, LGTM.

Yay! :partying_face: Thank you for your suggestions! :heart: 

> Do you need me to merge this for you?

Yes, please. I am not a maintainer, I don't have merge permissions.

https://github.com/llvm/llvm-project/pull/136190