Improve documentation to discourage non-zero returncodes (google#12955)

This PR adds a small documentation change to discourage the use of
non-zero return values.

Based on this issue google#11983,
Honggfuzz does not support other return values, while libFuzzer, in my
opinion unexpectedly, discards inputs for return values other than zero
which can cause creation of faulty harnesses. As a result I think it is
fair to discourage all uses of non-zero return values from
LLVMFuzzerTestOneInput.

docs/advanced-topics/ideal_integration.md

@@ -45,13 +45,13 @@ This makes it easy to maintain the fuzzers and minimizes breakages that can
 arise as source code changes over time.
 
 Make sure to fuzz the target locally for a small period of time to ensure that 
-it does not crash, hang, or run out of memory instantly. If you're having
-trouble, read about [what makes a good fuzz
+it does not crash, hang, or run out of memory instantly. Also make sure that the fuzzer can
+make at least some progress. If you're having trouble, read about [what makes a good fuzz
 target](https://github.com/google/fuzzing/blob/master/docs/good-fuzz-target.md).
 
 The interface between the [fuzz target](https://llvm.org/docs/LibFuzzer.html#fuzz-target)
 and the fuzzing engines is C, so you can use either C or C++ to implement the
-fuzz target.
+fuzz target. Make sure to not return values other than **zero** [^1].
 
 Examples: 
 [boringssl](https://github.com/google/boringssl/tree/master/fuzz),
@@ -64,6 +64,10 @@ Examples:
 [pcre2](https://vcs.pcre.org/pcre2/code/trunk/src/pcre2_fuzzsupport.c?view=markup),
 [ffmpeg](https://github.com/FFmpeg/FFmpeg/blob/master/tools/target_dec_fuzzer.c).
 
+[^1]: While LibFuzzer uses a non-zero value as a signal to discard inputs other fuzzers in
+use by OSS-Fuzz do not necessarily support this behavior. (Discarding inputs can be used
+to stop a fuzzer from exploring further, which should only be used with good reason.)
+
 ## Build support
 
 Many different build systems exist in the open-source world. The less OSS-Fuzz