Non-zero return values from LLVMFuzzerTestOneInput should be discouraged

[phi-go]

Hi, I was investigating a fuzzer target/harness that for some reason failed to create any meaningful coverage. Turns out that libFuzzer discards any inputs where the return value for LLVMFuzzerTestOneInput is non-zero and not only for -1. The documentation for libFuzzer only mentions 0 and -1, while other values are "undefined". While it seems that for Honggfuzz the return value is ignored #11983. This is not documented directly for OSS-Fuzz and as this seems like an easy and potentially severe mistake to make I think the documentation should be expanded.

The use of non-zero exit codes is also something that could potentially be detected and warned of automatically, is this something that you would be interested in?

[phi-go]

I added a PR to update the docs here #12955