[fuzzer] properly annotate fallthrough, add one more entry to FAQ

llvm-svn: 229880

lib/Fuzzer/FuzzerMutate.cpp

@@ -47,7 +47,7 @@ void Mutate(Unit *U, size_t MaxLen) {
       U->erase(U->begin() + rand() % U->size());
       break;
     }
-    // Fallthrough
+    [[clang::fallthrough]];
   case 1:
     if (U->size() < MaxLen) {
       U->insert(U->begin() + rand() % U->size(), RandCh());

lib/Fuzzer/README.txt

@@ -104,3 +104,9 @@ A.
     more) the speed benefit from the in-process fuzzer is negligible.
   * If the target library runs persistent threads (that outlive
     execution of one test) the fuzzing results will be unreliable.
+
+Q. So, what exactly this Fuzzer is good for?
+A. This Fuzzer might be a good choice for testing libraries that have relatively
+small inputs, each input takes < 1ms to run, and the library code is not expected
+to crash on invalid inputs.
+Examples: regular expression matchers, text or binary format parsers.