[docs] Try to clarify the FuzzingLLVM docs

Try to improve these docs based on some recent questions that were
sent to llvm-dev:

  http://lists.llvm.org/pipermail/llvm-dev/2018-August/125329.html

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@339949 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed