Documentation websites actually have some pretty good use-cases to being scalable to mobile, either because people will view documentation on a tablet or phone while developing, or due to people using smaller browser widths when developing to be able to display their IDE or terminal windows next to the documentation windows.
Well, the nice thing about mobile first is that the benefits extend to smaller browser widths as well. And just because you don't find a method appealing to use doesn't mean it's not a valid use case. I, for one, like to have all my documentation offline in ePub on a tablet, but that doesn't mean I assume it's the best method for everyone else (or even anyone else).
Plus, though it amy be an unintended consequence, mobile first shouldn't at all hurt the desktop experience, just make sure that mobile is not an afterthought once the design patterns are engrained and it's too late to modify a responsive experience.
