clarify that afl fuzz inputs are necessary and link to how to craft them

[cosmicexplorer]

Problem

• The book states that example inputs to AFL are not strictly required, which seems to be flatly false. I was not immediately able to identify any version of AFL which allows an empty set of input files, and the command line itself will refuse to start at all without providing a valid input directory with -i.
  • This is incredibly confusing, and the crate I'm contributing to has had a completely nonfunctional fuzz command line for months under the impression that they were doing it right somehow.
• The book goes on to state that "filenames don't matter" for input examples. This is a bit of a strange phrasing given that AFL (at least the version afl-fuzz++4.33c I have installed) does provide a -e ext flag that allows filtering inputs by file extension.
• Finally, there is no link to the afl README, which describes how to craft effective input examples, and would have allowed users to figure out for themselves that this docbook's statement about input examples not being strictly required was outdated and/or false.

Solution

• Provide a brief description of what AFL requires from input examples.
• Mention the -e ext flag.
• Link to the AFL README, and mention dictionaries for more complex input.
• Clarify that #[cfg(fuzzing)] is enabled for cargo afl build as well.

[smoelius]

Thank you, @cosmicexplorer. Your observations are spot-on and your changes definitely improve the tutorial. 🙏

[cosmicexplorer]

Thanks so much!! ^_^ I was really worried I was missing something serious. Really appreciate confirming that I had the right idea!