If nothing else at least explain what the 'FFML' is in the first place. The example is a good minor one, for describing fanon. I think the problem is more unprovoked Alphabet soup then the actual comment itself. If you list it as either: The FanFiction Mailing List (FFML) or FFML (FanFiction Mailing List)... then its not a in-joke thing, instead its an explained antidote. A brief paragraph explaining what the FFML is and how old it is, will take out the problems with the reference.
It is important that you don't have points where the guide you're writing starts to make constant obscure references and in-jokes... at the same time you should remove all your personal (explained in the document) experiences until its boring to read.
It is important that you don't have points where the guide you're writing starts to make constant obscure references and in-jokes... at the same time you should remove all your personal (explained in the document) experiences until its boring to read.