[llvm] Clarify conditions of `lit` exiting with exit code 1 (PR #136190)

[James Henderson via llvm-commits]

jh7370 wrote:

> Please let me know what you think about moving the definition of "failure" to "Test Status Results" section and referencing it from both "Exit Status" and the description of `--report-failures-only`?

This sounds like a good plan to me. We'll probably need to tweak some wording of the individual statuses.

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